diff --git a/include/swift/AST/Attr.h b/include/swift/AST/Attr.h
index 2e3d66f82fda3..11e80605567ea 100644
--- a/include/swift/AST/Attr.h
+++ b/include/swift/AST/Attr.h
@@ -364,6 +364,41 @@ class DeclAttribute : public AttributeBase {
 
     /// The opposite of ABIBreakingToRemove
     ABIStableToRemove = 1ull << 15,
+
+    /// Attribute should not be used in an \c \@abi attribute. Use for
+    /// attributes which cannot affect mangled names, even indirectly, and
+    /// which either don't affect ABI or where ABI-only declarations get their
+    /// behavior from their API counterpart.
+    ForbiddenInABIAttr = 1ull << 16,
+
+    /// Attribute can be used without restrictions in an \c \@abi attribute.
+    /// Use for attributes which affect mangled names but otherwise don't alter
+    /// the ABI, or ones where the \c ABIDeclChecker manually implements
+    /// special checking logic (e.g. because several different attributes
+    /// contribute to the same aspect of ABI in some complicated way).
+    UnconstrainedInABIAttr = 1ull << 17,
+
+    /// Attribute can be used in an \c \@abi attribute, but must match
+    /// equivalent on API decl. Use for attributes which affect both mangled
+    /// names and other parts of the ABI such that the declaration can only be
+    /// valid if they match. 
+    EquivalentInABIAttr = 1ull << 18,
+
+    /// Attribute can be used in an \c \@abi attribute, but must match
+    /// equivalent on API decl; if omitted, API decl's attribute will be 
+    /// cloned. Use where you would want to use \c EquivalentInABIAttr but 
+    /// repeating the attribute is judged too burdensome.
+    InferredInABIAttr = 1ull << 19,
+
+    /// Use for attributes which are \em only valid on declarations that cannot
+    /// have an \c @abi attribute, such as \c ImportDecl .
+    UnreachableInABIAttr = 1ull << 20,
+  };
+
+  enum : uint64_t {
+    InABIAttrMask = ForbiddenInABIAttr | UnconstrainedInABIAttr
+                  | EquivalentInABIAttr | InferredInABIAttr
+                  | UnreachableInABIAttr
   };
 
   LLVM_READNONE
@@ -549,6 +584,11 @@ class DeclAttribute : public AttributeBase {
 
   /// Determine whether we can clone this attribute.
   bool canClone() const;
+
+  /// Determine if this attribute and \p other are "the same", as in, they
+  /// would have the same effect on \p attachedTo were they attached to it. A
+  /// clone should always be equivalent to the original.
+  bool isEquivalent(const DeclAttribute *other, Decl *attachedTo) const;
 };
 
 #define UNIMPLEMENTED_CLONE(AttrType)    \
@@ -582,6 +622,11 @@ class SimpleDeclAttr : public DeclAttribute {
     return new (ctx) SimpleDeclAttr<Kind>(
         AtLoc, Range.Start, isImplicit());
   }
+
+  bool isEquivalent(const SimpleDeclAttr<Kind> *other, Decl *attachedTo) const {
+    // True by definition, since there's nothing to this other than its kind.
+    return true;
+  }
 };
 
 // Declare typedefs for all of the simple declaration attributes.
@@ -614,6 +659,10 @@ class SILGenNameAttr : public DeclAttribute {
   SILGenNameAttr *clone(ASTContext &ctx) const {
     return new (ctx) SILGenNameAttr(Name, Raw, AtLoc, Range, isImplicit());
   }
+
+  bool isEquivalent(const SILGenNameAttr *other, Decl *attachedTo) const {
+    return Name == other->Name && Raw == other->Raw;
+  }
 };
 
 /// Defines the @_section attribute.
@@ -636,6 +685,10 @@ class SectionAttr : public DeclAttribute {
   SectionAttr *clone(ASTContext &ctx) const {
     return new (ctx) SectionAttr(Name, AtLoc, Range, isImplicit());
   }
+
+  bool isEquivalent(const SectionAttr *other, Decl *attachedTo) const {
+    return Name == other->Name;
+  }
 };
 
 /// Defines the @_cdecl attribute.
@@ -658,6 +711,10 @@ class CDeclAttr : public DeclAttribute {
   CDeclAttr *clone(ASTContext &ctx) const {
     return new (ctx) CDeclAttr(Name, AtLoc, Range, isImplicit());
   }
+
+  bool isEquivalent(const CDeclAttr *other, Decl *attachedTo) const {
+    return Name == other->Name;
+  }
 };
 
 /// Defines the @_semantics attribute.
@@ -681,6 +738,10 @@ class SemanticsAttr : public DeclAttribute {
   SemanticsAttr *clone(ASTContext &ctx) const {
     return new (ctx) SemanticsAttr(Value, AtLoc, Range, isImplicit());
   }
+
+  bool isEquivalent(const SemanticsAttr *other, Decl *attachedTo) const {
+    return Value == other->Value;
+  }
 };
 
 /// Defines the @_alignment attribute.
@@ -701,6 +762,10 @@ class AlignmentAttr : public DeclAttribute {
   AlignmentAttr *clone(ASTContext &ctx) const {
     return new (ctx) AlignmentAttr(getValue(), AtLoc, Range, isImplicit());
   }
+
+  bool isEquivalent(const AlignmentAttr *other, Decl *attachedTo) const {
+    return getValue() == other->getValue();
+  }
 };
 
 /// Defines the @_swift_native_objc_runtime_base attribute.
@@ -732,6 +797,11 @@ class SwiftNativeObjCRuntimeBaseAttr : public DeclAttribute {
     return new (ctx) SwiftNativeObjCRuntimeBaseAttr(
         BaseClassName, AtLoc, Range, isImplicit());
   }
+
+  bool isEquivalent(const SwiftNativeObjCRuntimeBaseAttr *other,
+                    Decl *attachedTo) const {
+    return BaseClassName == other->BaseClassName;
+  }
 };
 
 /// Defines the @available attribute.
@@ -914,6 +984,8 @@ class AvailableAttr : public DeclAttribute {
     return clone(C, isImplicit());
   }
 
+  bool isEquivalent(const AvailableAttr *other, Decl *attachedTo) const;
+
   static bool classof(const DeclAttribute *DA) {
     return DA->getKind() == DeclAttrKind::Available;
   }
@@ -1081,6 +1153,16 @@ class ObjCAttr final : public DeclAttribute,
   /// original without source location information.
   ObjCAttr *clone(ASTContext &context) const;
 
+  bool isEquivalent(const ObjCAttr *other, Decl *attachedTo) const {
+    std::optional<ObjCSelector> thisName, otherName;
+    if (hasName() && !isNameImplicit())
+      thisName = getName();
+    if (other->hasName() && !other->isNameImplicit())
+      otherName = other->getName();
+
+    return thisName == otherName;
+  }
+
   static bool classof(const DeclAttribute *DA) {
     return DA->getKind() == DeclAttrKind::ObjC;
   }
@@ -1111,6 +1193,10 @@ class PrivateImportAttr final
     return new (ctx) PrivateImportAttr(
         AtLoc, Range, SourceFile, SourceRange(), isImplicit());
   }
+
+  bool isEquivalent(const PrivateImportAttr *other, Decl *attachedTo) const {
+    return getSourceFile() == other->getSourceFile();
+  }
 };
 
 /// The @_dynamicReplacement(for:) attribute.
@@ -1190,6 +1276,10 @@ class DynamicReplacementAttr final
   }
 
   UNIMPLEMENTED_CLONE(DynamicReplacementAttr)
+
+  bool isEquivalent(const DynamicReplacementAttr *other, Decl *attachedTo) const {
+    return getReplacedFunctionName() == other->getReplacedFunctionName();
+  }
 };
 
 /// The \c @_typeEraser(TypeEraserType) attribute.
@@ -1247,6 +1337,8 @@ class TypeEraserAttr final : public DeclAttribute {
   }
 
   UNIMPLEMENTED_CLONE(TypeEraserAttr)
+
+  bool isEquivalent(const TypeEraserAttr *other, Decl *attachedTo) const;
 };
 
 /// Represents any sort of access control modifier.
@@ -1286,6 +1378,10 @@ class AccessControlAttr : public AbstractAccessControlAttr {
   AccessControlAttr *clone(ASTContext &ctx) const {
     return new (ctx) AccessControlAttr(AtLoc, Range, getAccess(), isImplicit());
   }
+
+  bool isEquivalent(const AccessControlAttr *other, Decl *attachedTo) const {
+    return getAccess() == other->getAccess();
+  }
 };
 
 /// Represents a 'private', 'internal', or 'public' marker for a setter on a
@@ -1305,6 +1401,10 @@ class SetterAccessAttr : public AbstractAccessControlAttr {
   SetterAccessAttr *clone(ASTContext &ctx) const {
     return new (ctx) SetterAccessAttr(AtLoc, Range, getAccess(), isImplicit());
   }
+
+  bool isEquivalent(const SetterAccessAttr *other, Decl *attachedTo) const {
+    return getAccess() == other->getAccess();
+  }
 };
 
 /// SPI attribute applied to both decls and imports.
@@ -1341,6 +1441,8 @@ class SPIAccessControlAttr final : public DeclAttribute,
   static bool classof(const DeclAttribute *DA) {
     return DA->getKind() == DeclAttrKind::SPIAccessControl;
   }
+
+   bool isEquivalent(const SPIAccessControlAttr *other, Decl *attachedTo) const;
 };
 
 /// Represents an inline attribute.
@@ -1363,6 +1465,10 @@ class InlineAttr : public DeclAttribute {
   InlineAttr *clone(ASTContext &ctx) const {
     return new (ctx) InlineAttr(AtLoc, Range, getKind(), isImplicit());
   }
+
+  bool isEquivalent(const InlineAttr *other, Decl *attachedTo) const {
+    return getKind() == other->getKind();
+  }
 };
 
 /// Represents the optimize attribute.
@@ -1387,6 +1493,10 @@ class OptimizeAttr : public DeclAttribute {
   OptimizeAttr *clone(ASTContext &ctx) const {
     return new (ctx) OptimizeAttr(AtLoc, Range, getMode(), isImplicit());
   }
+
+  bool isEquivalent(const OptimizeAttr *other, Decl *attachedTo) const {
+    return getMode() == other->getMode();
+  }
 };
 
 /// Represents the exclusivity attribute.
@@ -1418,6 +1528,10 @@ class ExclusivityAttr : public DeclAttribute {
   ExclusivityAttr *clone(ASTContext &ctx) const {
     return new (ctx) ExclusivityAttr(AtLoc, Range, getMode(), isImplicit());
   }
+
+  bool isEquivalent(const ExclusivityAttr *other, Decl *attachedTo) const {
+    return getMode() == other->getMode();
+  }
 };
 
 /// Represents the side effects attribute.
@@ -1464,6 +1578,14 @@ class EffectsAttr : public DeclAttribute {
     }
     return new (ctx) EffectsAttr(getKind());
   }
+
+  bool isEquivalent(const EffectsAttr *other, Decl *attachedTo) const {
+    if (getKind() != other->getKind())
+      return false;
+    if (getKind() == EffectsKind::Custom)
+      return getCustomString() == other->getCustomString();
+    return true;
+  }
 };
 
 
@@ -1492,6 +1614,11 @@ class ReferenceOwnershipAttr : public DeclAttribute {
   static bool classof(const DeclAttribute *DA) {
     return DA->getKind() == DeclAttrKind::ReferenceOwnership;
   }
+
+  bool isEquivalent(const ReferenceOwnershipAttr *other,
+                    Decl *attachedTo) const {
+    return get() == other->get();
+  }
 };
 
 /// Defines the attribute that we use to model documentation comments.
@@ -1515,6 +1642,10 @@ class RawDocCommentAttr : public DeclAttribute {
   RawDocCommentAttr *clone(ASTContext &ctx) const {
     return new (ctx) RawDocCommentAttr(CommentRange, isImplicit());
   }
+
+  bool isEquivalent(const RawDocCommentAttr *other, Decl *attachedTo) const {
+    return getCommentRange() == other->getCommentRange();
+  }
 };
 
 /// An attribute applied to a CoreFoundation class that is toll-free bridged to
@@ -1542,6 +1673,10 @@ class ObjCBridgedAttr : public DeclAttribute {
   ObjCBridgedAttr *clone(ASTContext &ctx) const {
     return new (ctx) ObjCBridgedAttr(ObjCClass);
   }
+
+  bool isEquivalent(const ObjCBridgedAttr *other, Decl *attachedTo) const {
+    return getObjCClass() == other->getObjCClass();
+  }
 };
 
 /// An attribute that specifies a synthesized conformance of a known
@@ -1586,6 +1721,13 @@ class SynthesizedProtocolAttr : public DeclAttribute {
     return new (ctx) SynthesizedProtocolAttr(
         protocol, getLazyLoader(), isUnchecked());
   }
+
+  bool isEquivalent(const SynthesizedProtocolAttr *other,
+                    Decl *attachedTo) const {
+    return isUnchecked() == other->isUnchecked()
+            && getProtocol() == other->getProtocol()
+            && getLazyLoader() == other->getLazyLoader();
+  }
 };
 
 /// The @_specialize attribute, which forces specialization on the specified
@@ -1719,6 +1861,8 @@ class SpecializeAttr final
   }
 
   UNIMPLEMENTED_CLONE(SpecializeAttr)
+
+  bool isEquivalent(const SpecializeAttr *other, Decl *attachedTo) const;
 };
 
 class StorageRestrictionsAttr final
@@ -1763,19 +1907,24 @@ class StorageRestrictionsAttr final
   }
 
   UNIMPLEMENTED_CLONE(StorageRestrictionsAttr)
+
+  bool isEquivalent(const StorageRestrictionsAttr *other,
+                    Decl *attachedTo) const;
 };
 
 /// The @_implements attribute, which treats a decl as the implementation for
 /// some named protocol requirement (but otherwise not-visible by that name).
 class ImplementsAttr : public DeclAttribute {
-  TypeRepr *TyR;
+  /// If constructed by the \c create() variant with a TypeRepr, the TypeRepr;
+  /// if constructed by the \c create() variant with a DeclContext and
+  /// ProtocolDecl, the DeclContext.
+  llvm::PointerUnion<TypeRepr *, DeclContext *> TyROrDC;
   DeclName MemberName;
   DeclNameLoc MemberNameLoc;
 
   ImplementsAttr(SourceLoc atLoc, SourceRange Range,
-                 TypeRepr *TyR,
-                 DeclName MemberName,
-                 DeclNameLoc MemberNameLoc);
+                 llvm::PointerUnion<TypeRepr *, DeclContext *> TyROrDC,
+                 DeclName MemberName, DeclNameLoc MemberNameLoc);
 
 public:
   static ImplementsAttr *create(ASTContext &Ctx, SourceLoc atLoc,
@@ -1795,7 +1944,9 @@ class ImplementsAttr : public DeclAttribute {
   /// otherwise `nullopt`. This should only be used for dumping.
   std::optional<ProtocolDecl *> getCachedProtocol(DeclContext *dc) const;
 
-  TypeRepr *getProtocolTypeRepr() const { return TyR; }
+  TypeRepr *getProtocolTypeRepr() const {
+    return TyROrDC.dyn_cast<TypeRepr *>();
+  }
 
   DeclName getMemberName() const { return MemberName; }
   DeclNameLoc getMemberNameLoc() const { return MemberNameLoc; }
@@ -1806,9 +1957,15 @@ class ImplementsAttr : public DeclAttribute {
 
   /// Create a copy of this attribute.
   ImplementsAttr *clone(ASTContext &ctx) const {
-    return new (ctx) ImplementsAttr(
-        AtLoc, Range, TyR, getMemberName(), getMemberNameLoc());
+    if (auto tyR = getProtocolTypeRepr()) {
+      return create(ctx, AtLoc, Range, tyR, getMemberName(),
+                    getMemberNameLoc());
+    }
+    auto dc = TyROrDC.dyn_cast<DeclContext *>();
+    return create(dc, getProtocol(dc), getMemberName());
   }
+
+  bool isEquivalent(const ImplementsAttr *other, Decl *attachedTo) const;
 };
 
 /// A limited variant of \c \@objc that's used for classes with generic ancestry.
@@ -1837,6 +1994,10 @@ class ObjCRuntimeNameAttr : public DeclAttribute {
   ObjCRuntimeNameAttr *clone(ASTContext &ctx) const {
     return new (ctx) ObjCRuntimeNameAttr(Name, AtLoc, Range, isImplicit());
   }
+
+  bool isEquivalent(const ObjCRuntimeNameAttr *other, Decl *attachedTo) const {
+    return Name == other->Name;
+  }
 };
 
 /// Attribute that specifies a protocol conformance that has been restated
@@ -1860,6 +2021,11 @@ class RestatedObjCConformanceAttr : public DeclAttribute {
   RestatedObjCConformanceAttr *clone(ASTContext &ctx) const {
     return new (ctx) RestatedObjCConformanceAttr(Proto);
   }
+
+  bool isEquivalent(const RestatedObjCConformanceAttr *other,
+                    Decl *attachedTo) const {
+    return Proto == other->Proto;
+  }
 };
 
 /// Attached to type declarations synthesized by the Clang importer.
@@ -1927,6 +2093,12 @@ class ClangImporterSynthesizedTypeAttr : public DeclAttribute {
     return new (ctx) ClangImporterSynthesizedTypeAttr(
         originalTypeName, getKind());
   }
+
+  bool isEquivalent(const ClangImporterSynthesizedTypeAttr *other,
+                    Decl *attachedTo) const {
+    return getKind() == other->getKind()
+            && originalTypeName == other->originalTypeName;
+  }
 };
 
 /// Defines a custom attribute.
@@ -1998,6 +2170,8 @@ class CustomAttr final : public DeclAttribute {
 
   bool canClone() const { return argList == nullptr; }
 
+  bool isEquivalent(const CustomAttr *other, Decl *attachedTo) const;
+
 private:
   friend class CustomAttrNominalRequest;
   void resetTypeInformation(TypeExpr *repr);
@@ -2036,6 +2210,11 @@ class ProjectedValuePropertyAttr : public DeclAttribute {
     return new (ctx) ProjectedValuePropertyAttr(
         ProjectionPropertyName, AtLoc, Range, isImplicit());
   }
+
+  bool isEquivalent(const ProjectedValuePropertyAttr *other,
+                    Decl *attachedTo) const {
+    return ProjectionPropertyName == other->ProjectionPropertyName;
+  }
 };
 
 /// Describes a symbol that was originally defined in another module. For
@@ -2101,6 +2280,14 @@ class OriginallyDefinedInAttr: public DeclAttribute {
         AtLoc, Range, ManglingModuleName, LinkerModuleName,
         Platform, MovedVersion, isImplicit());
   }
+
+  bool isEquivalent(const OriginallyDefinedInAttr *other,
+                    Decl *attachedTo) const {
+    return ManglingModuleName == other->ManglingModuleName
+            && LinkerModuleName == other->LinkerModuleName
+            && Platform == other->Platform
+            && MovedVersion == other->MovedVersion;
+  }
 };
 
 /// Attribute that marks a function as differentiable.
@@ -2250,6 +2437,11 @@ class DifferentiableAttr final
   }
 
   UNIMPLEMENTED_CLONE(DifferentiableAttr)
+
+  bool isEquivalent(const DifferentiableAttr *other, Decl *attachedTo) const {
+    // Not properly implemented (very complex and not currently needed)
+    return false;
+  }
 };
 
 /// A declaration name with location.
@@ -2391,6 +2583,11 @@ class DerivativeAttr final
   }
 
   UNIMPLEMENTED_CLONE(DerivativeAttr)
+
+  bool isEquivalent(const DerivativeAttr *other, Decl *attachedTo) const {
+    // Not properly implemented (very complex and not currently needed)
+    return false;
+  }
 };
 
 /// The `@transpose(of:)` attribute registers a function as a transpose of
@@ -2477,6 +2674,11 @@ class TransposeAttr final
   }
 
   UNIMPLEMENTED_CLONE(TransposeAttr)
+
+  bool isEquivalent(const TransposeAttr *other, Decl *attachedTo) const {
+    // Not properly implemented (very complex and not currently needed)
+    return false;
+  }
 };
 
 enum class NonSendableKind : uint8_t {
@@ -2512,6 +2714,10 @@ class NonSendableAttr : public DeclAttribute {
   NonSendableAttr *clone(ASTContext &ctx) const {
     return new (ctx) NonSendableAttr(AtLoc, Range, Specificity, isImplicit());
   }
+
+  bool isEquivalent(const NonSendableAttr *other, Decl *attachedTo) const {
+    return Specificity == other->Specificity;
+  }
 };
 
 /// The @_unavailableFromAsync attribute, used to make function declarations
@@ -2539,6 +2745,11 @@ class UnavailableFromAsyncAttr : public DeclAttribute {
     return new (ctx) UnavailableFromAsyncAttr(
         Message, AtLoc, Range, isImplicit());
   }
+
+  bool isEquivalent(const UnavailableFromAsyncAttr *other,
+                    Decl *attachedTo) const {
+    return Message == other->Message;
+  }
 };
 
 /// The `@backDeployed(...)` attribute, used to make function declarations
@@ -2569,6 +2780,10 @@ class BackDeployedAttr : public DeclAttribute {
     return new (ctx) BackDeployedAttr(
         AtLoc, Range, Platform, Version, isImplicit());
   }
+
+  bool isEquivalent(const BackDeployedAttr *other, Decl *attachedTo) const {
+    return Platform == other->Platform && Version == Version;
+  }
 };
 
 /// Defines the `@_expose` attribute, used to expose declarations in the
@@ -2602,6 +2817,10 @@ class ExposeAttr : public DeclAttribute {
     return new (ctx) ExposeAttr(
         Name, AtLoc, Range, getExposureKind(), isImplicit());
   }
+
+  bool isEquivalent(const ExposeAttr *other, Decl *attachedTo) const {
+    return Name == other->Name && getExposureKind() == other->getExposureKind();
+  }
 };
 
 /// Define the `@_extern` attribute, used to import external declarations in
@@ -2657,6 +2876,11 @@ class ExternAttr : public DeclAttribute {
         ModuleName, Name, AtLoc, getLParenLoc(), getRParenLoc(), Range,
         getExternKind(), isImplicit());
   }
+
+  bool isEquivalent(const ExternAttr *other, Decl *attachedTo) const {
+    return getExternKind() == other->getExternKind()
+            && ModuleName == other->ModuleName && Name == other->Name;
+  }
 };
 
 /// The `@_documentation(...)` attribute, used to override a symbol's visibility
@@ -2685,6 +2909,10 @@ class DocumentationAttr: public DeclAttribute {
     return new (ctx) DocumentationAttr(
         AtLoc, Range, Metadata, Visibility, isImplicit());
   }
+
+  bool isEquivalent(const DocumentationAttr *other, Decl *attachedTo) const {
+    return Metadata == other->Metadata && Visibility == other->Visibility;
+  }
 };
 
 class ObjCImplementationAttr final : public DeclAttribute {
@@ -2739,6 +2967,12 @@ class ObjCImplementationAttr final : public DeclAttribute {
         CategoryName, AtLoc, Range, isEarlyAdopter(), isImplicit(),
         isCategoryNameInvalid());
   }
+
+  bool isEquivalent(const ObjCImplementationAttr *other,
+                    Decl *attachedTo) const {
+    // Ignoring `CategoryName` as it gets copied to @objc
+    return isEarlyAdopter() == other->isEarlyAdopter();
+  }
 };
 
 /// Represents nonisolated modifier.
@@ -2764,6 +2998,10 @@ class NonisolatedAttr final : public DeclAttribute {
   NonisolatedAttr *clone(ASTContext &ctx) const {
     return new (ctx) NonisolatedAttr(AtLoc, Range, isUnsafe(), isImplicit());
   }
+
+  bool isEquivalent(const NonisolatedAttr *other, Decl *attachedTo) const {
+    return isUnsafe() == other->isUnsafe();
+  }
 };
 
 /// A macro role attribute, spelled with either @attached or @freestanding,
@@ -2817,6 +3055,11 @@ class MacroRoleAttr final
   }
 
   UNIMPLEMENTED_CLONE(MacroRoleAttr)
+
+  bool isEquivalent(const MacroRoleAttr *other, Decl *attachedTo) const {
+    // Unimplemented: complex and not immediately needed.
+    return true;
+  }
 };
 
 /// Specifies the raw storage used by a type.
@@ -2920,6 +3163,8 @@ class RawLayoutAttr final : public DeclAttribute {
   }
 
   UNIMPLEMENTED_CLONE(RawLayoutAttr)
+
+  bool isEquivalent(const RawLayoutAttr *other, Decl *attachedTo) const;
 };
 
 class LifetimeAttr final : public DeclAttribute {
@@ -2945,6 +3190,8 @@ class LifetimeAttr final : public DeclAttribute {
   LifetimeAttr *clone(ASTContext &ctx) const {
     return new (ctx) LifetimeAttr(AtLoc, Range, isImplicit(), entry);
   }
+
+  bool isEquivalent(const LifetimeAttr *other, Decl *attachedTo) const;
 };
 
 /// Predicate used to filter MatchingAttributeRange.
@@ -2993,6 +3240,9 @@ class AllowFeatureSuppressionAttr final
   }
 
   UNIMPLEMENTED_CLONE(AllowFeatureSuppressionAttr)
+
+  bool isEquivalent(const AllowFeatureSuppressionAttr *other,
+                    Decl *attachedTo) const;
 };
 
 /// Defines the @abi attribute.
@@ -3018,6 +3268,11 @@ class ABIAttr : public DeclAttribute {
   }
 
   UNIMPLEMENTED_CLONE(ABIAttr)
+
+  bool isEquivalent(const ABIAttr *other, Decl *attachedTo) const {
+    // Unsupported: tricky to implement and unneeded.
+    return true;
+  }
 };
 
 class ExecutionAttr : public DeclAttribute {
@@ -3042,6 +3297,10 @@ class ExecutionAttr : public DeclAttribute {
   }
 
   UNIMPLEMENTED_CLONE(ExecutionAttr)
+
+  bool isEquivalent(const ExecutionAttr *other, Decl *attachedTo) const {
+    return getBehavior() == other->getBehavior();
+  }
 };
 
 /// Attributes that may be applied to declarations.
diff --git a/include/swift/AST/AttrKind.h b/include/swift/AST/AttrKind.h
index 1f4a784c70510..d56edbc4b6e31 100644
--- a/include/swift/AST/AttrKind.h
+++ b/include/swift/AST/AttrKind.h
@@ -145,6 +145,8 @@ enum class DeclAttrKind : unsigned {
 #include "swift/AST/DeclAttr.def"
 };
 
+StringRef getDeclAttrKindID(DeclAttrKind kind);
+
 enum : unsigned {
   NumDeclAttrKinds = static_cast<unsigned>(DeclAttrKind::Last_DeclAttr) + 1,
   NumDeclAttrKindBits = countBitsUsed(NumDeclAttrKinds - 1),
diff --git a/include/swift/AST/Decl.h b/include/swift/AST/Decl.h
index ab3d8fde31c39..0544e49bed8b6 100644
--- a/include/swift/AST/Decl.h
+++ b/include/swift/AST/Decl.h
@@ -1041,6 +1041,10 @@ class alignas(1 << DeclAlignInBits) Decl : public ASTAllocated<Decl>, public Swi
   /// from source code.
   void attachParsedAttrs(DeclAttributes attrs);
 
+  /// Retrieve the custom name in the \c @objc attribute, if present.
+  std::optional<ObjCSelector>
+  getExplicitObjCName(bool allowInvalid = false) const;
+
   /// True if this declaration provides an implementation for an imported
   /// Objective-C declaration. This implies various restrictions and special
   /// behaviors for it and, if it's an extension, its members.
@@ -5919,6 +5923,13 @@ class AbstractStorageDecl : public ValueDecl {
   /// Return the interface type of the stored value.
   Type getValueInterfaceType() const;
 
+  /// Retrieve the source range of the variable type, or an invalid range if the
+  /// variable's type is not explicitly written in the source.
+  ///
+  /// Only for use in diagnostics.  It is not always possible to always
+  /// precisely point to the variable type because of type aliases.
+  SourceRange getTypeSourceRangeForDiagnostics() const;
+
   /// Determine how this storage is implemented.
   StorageImplInfo getImplInfo() const;
 
@@ -6353,13 +6364,6 @@ class VarDecl : public AbstractStorageDecl {
   /// and not just getInterfaceType().
   Type getTypeInContext() const;
 
-  /// Retrieve the source range of the variable type, or an invalid range if the
-  /// variable's type is not explicitly written in the source.
-  ///
-  /// Only for use in diagnostics.  It is not always possible to always
-  /// precisely point to the variable type because of type aliases.
-  SourceRange getTypeSourceRangeForDiagnostics() const;
-
   /// Determine the mutability of this variable declaration when
   /// accessed from a given declaration context.
   StorageMutability mutability(
diff --git a/include/swift/AST/DeclAttr.def b/include/swift/AST/DeclAttr.def
index a7cb4d7d4eb59..654d9cf221688 100644
--- a/include/swift/AST/DeclAttr.def
+++ b/include/swift/AST/DeclAttr.def
@@ -63,142 +63,142 @@
 // leave an "Unused 'NNN'" comment so the unused code is obvious. 
 DECL_ATTR(_silgen_name, SILGenName,
   OnAbstractFunction | OnVar,
-  LongAttribute | UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  LongAttribute | UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   0)
 
 DECL_ATTR(available, Available,
   OnAbstractFunction | OnAssociatedType | OnGenericType | OnVar | OnSubscript | OnEnumElement | OnMacro | OnExtension,
-  AllowMultipleAttributes | LongAttribute | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  AllowMultipleAttributes | LongAttribute | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   1)
 
 CONTEXTUAL_SIMPLE_DECL_ATTR(final, Final,
   OnClass | OnFunc | OnAccessor | OnVar | OnSubscript,
-  DeclModifier | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove,
+  DeclModifier | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove | UnconstrainedInABIAttr,
   2)
 
 DECL_ATTR(objc, ObjC,
   OnAbstractFunction | OnClass | OnProtocol | OnExtension | OnVar | OnSubscript | OnEnum | OnEnumElement,
-  ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove,
+  ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   3)
 
 CONTEXTUAL_SIMPLE_DECL_ATTR(required, Required,
   OnConstructor,
-  DeclModifier | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  DeclModifier | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | EquivalentInABIAttr,
   4)
 
 CONTEXTUAL_SIMPLE_DECL_ATTR(optional, Optional,
   OnConstructor | OnFunc | OnAccessor | OnVar | OnSubscript,
-  DeclModifier | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  DeclModifier | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   5)
 
 SIMPLE_DECL_ATTR(dynamicCallable, DynamicCallable,
   OnNominalType,
-  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   6)
 
 SIMPLE_DECL_ATTR(main, MainType,
   OnClass | OnStruct | OnEnum | OnExtension,
-  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   7)
 
 SIMPLE_DECL_ATTR(_exported, Exported,
   OnImport,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   8)
 
 SIMPLE_DECL_ATTR(dynamicMemberLookup, DynamicMemberLookup,
   OnNominalType,
-  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   9)
 
 SIMPLE_DECL_ATTR(NSCopying, NSCopying,
   OnVar,
-  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   10)
 
 SIMPLE_DECL_ATTR(IBAction, IBAction,
   OnFunc,
-  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   11)
 
 SIMPLE_DECL_ATTR(IBDesignable, IBDesignable,
   OnClass | OnExtension,
-  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   12)
 
 SIMPLE_DECL_ATTR(IBInspectable, IBInspectable,
   OnVar,
-  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   13)
 
 SIMPLE_DECL_ATTR(IBOutlet, IBOutlet,
   OnVar,
-  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   14)
 
 SIMPLE_DECL_ATTR(NSManaged, NSManaged,
   OnVar | OnFunc | OnAccessor,
-  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   15)
 
 CONTEXTUAL_SIMPLE_DECL_ATTR(lazy, Lazy,
   OnVar,
-  DeclModifier | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  DeclModifier | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | InferredInABIAttr,
   16)
 
 SIMPLE_DECL_ATTR(LLDBDebuggerFunction, LLDBDebuggerFunction,
   OnFunc,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   17)
 
 SIMPLE_DECL_ATTR(UIApplicationMain, UIApplicationMain,
   OnClass,
-  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   18)
 
 SIMPLE_DECL_ATTR(unsafe_no_objc_tagged_pointer, UnsafeNoObjCTaggedPointer,
   OnProtocol,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   19)
 
 DECL_ATTR(inline, Inline,
   OnVar | OnSubscript | OnAbstractFunction,
-  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | InferredInABIAttr,
   20)
 
 DECL_ATTR(_semantics, Semantics,
   OnAbstractFunction | OnSubscript | OnNominalType | OnVar,
-  AllowMultipleAttributes | UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  AllowMultipleAttributes | UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnconstrainedInABIAttr,
   21)
 
 CONTEXTUAL_SIMPLE_DECL_ATTR(dynamic, Dynamic,
   OnFunc | OnAccessor | OnVar | OnSubscript | OnConstructor,
-  DeclModifier | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove,
+  DeclModifier | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   22)
 
 CONTEXTUAL_SIMPLE_DECL_ATTR(infix, Infix,
   OnFunc | OnOperator,
-  DeclModifier | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  DeclModifier | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   23)
 
 CONTEXTUAL_SIMPLE_DECL_ATTR(prefix, Prefix,
   OnFunc | OnOperator,
-  DeclModifier | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  DeclModifier | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnconstrainedInABIAttr,
   24)
 
 CONTEXTUAL_SIMPLE_DECL_ATTR(postfix, Postfix,
   OnFunc | OnOperator,
-  DeclModifier | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  DeclModifier | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnconstrainedInABIAttr,
   25)
 
 SIMPLE_DECL_ATTR(_transparent, Transparent,
   OnFunc | OnAccessor | OnConstructor | OnVar | OnDestructor,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | InferredInABIAttr,
   26)
 
 SIMPLE_DECL_ATTR(requires_stored_property_inits, RequiresStoredPropertyInits,
   OnClass,
-  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   27)
 
 // Unused '28'
@@ -206,37 +206,37 @@ SIMPLE_DECL_ATTR(requires_stored_property_inits, RequiresStoredPropertyInits,
 
 SIMPLE_DECL_ATTR(nonobjc, NonObjC,
   OnExtension | OnFunc | OnAccessor | OnVar | OnSubscript | OnConstructor,
-  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   30)
 
 SIMPLE_DECL_ATTR(_fixed_layout, FixedLayout,
   OnVar | OnClass | OnStruct | OnProtocol,
-  UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   31)
 
 SIMPLE_DECL_ATTR(inlinable, Inlinable,
   OnVar | OnSubscript | OnAbstractFunction,
-  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | InferredInABIAttr,
   32)
 
 DECL_ATTR(_specialize, Specialize,
   OnConstructor | OnFunc | OnAccessor,
-  AllowMultipleAttributes | LongAttribute | UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  AllowMultipleAttributes | LongAttribute | UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   33)
 
 SIMPLE_DECL_ATTR(objcMembers, ObjCMembers,
   OnClass,
-  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   34)
 
 CONTEXTUAL_SIMPLE_DECL_ATTR(_compilerInitialized, CompilerInitialized,
   OnVar,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   35)
 
 SIMPLE_DECL_ATTR(_lexicalLifetimes, LexicalLifetimes,
   OnFunc,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   36)
 
 // Unused '37'
@@ -245,37 +245,37 @@ SIMPLE_DECL_ATTR(_lexicalLifetimes, LexicalLifetimes,
 
 CONTEXTUAL_SIMPLE_DECL_ATTR(__consuming, LegacyConsuming,
   OnFunc | OnAccessor,
-  DeclModifier | UserInaccessible | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  DeclModifier | UserInaccessible | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnconstrainedInABIAttr,
   40)
 
 CONTEXTUAL_SIMPLE_DECL_ATTR(mutating, Mutating,
   OnFunc | OnAccessor,
-  DeclModifier | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  DeclModifier | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnconstrainedInABIAttr,
   41)
 
 CONTEXTUAL_SIMPLE_DECL_ATTR(nonmutating, NonMutating,
   OnFunc | OnAccessor,
-  DeclModifier | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  DeclModifier | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnconstrainedInABIAttr,
   42)
 
 CONTEXTUAL_SIMPLE_DECL_ATTR(convenience, Convenience,
   OnConstructor,
-  DeclModifier | NotSerialized | ABIBreakingToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  DeclModifier | NotSerialized | ABIBreakingToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | EquivalentInABIAttr,
   43)
 
 CONTEXTUAL_SIMPLE_DECL_ATTR(override, Override,
   OnFunc | OnAccessor | OnVar | OnSubscript | OnConstructor | OnAssociatedType,
-  DeclModifier | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  DeclModifier | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   44)
 
 SIMPLE_DECL_ATTR(_hasStorage, HasStorage,
   OnVar,
-  UserInaccessible | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   45)
 
 DECL_ATTR(private, AccessControl,
   OnFunc | OnAccessor | OnExtension | OnGenericType | OnVar | OnSubscript | OnConstructor | OnMacro | OnImport,
-  DeclModifier | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  DeclModifier | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   46)
 DECL_ATTR_ALIAS(fileprivate, AccessControl)
 DECL_ATTR_ALIAS(internal, AccessControl)
@@ -285,540 +285,541 @@ CONTEXTUAL_DECL_ATTR_ALIAS(open, AccessControl)
 
 DECL_ATTR(__setter_access, SetterAccess,
   OnVar | OnSubscript,
-  DeclModifier | RejectByParser | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  DeclModifier | RejectByParser | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   47)
 
 DECL_ATTR(__raw_doc_comment, RawDocComment,
   OnAnyDecl,
-  UserInaccessible | RejectByParser | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | RejectByParser | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnconstrainedInABIAttr,
   48)
 
 CONTEXTUAL_DECL_ATTR(weak, ReferenceOwnership,
   OnVar,
-  DeclModifier | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  DeclModifier | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   49)
 CONTEXTUAL_DECL_ATTR_ALIAS(unowned, ReferenceOwnership)
 
 DECL_ATTR(_effects, Effects,
   OnAbstractFunction,
-  AllowMultipleAttributes | UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  AllowMultipleAttributes | UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   50)
 
 DECL_ATTR(__objc_bridged, ObjCBridged,
   OnClass,
-  UserInaccessible | RejectByParser | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | RejectByParser | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   51)
 
 SIMPLE_DECL_ATTR(NSApplicationMain, NSApplicationMain,
   OnClass,
-  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   52)
 
 SIMPLE_DECL_ATTR(_objc_non_lazy_realization, ObjCNonLazyRealization,
   OnClass,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   53)
 
 DECL_ATTR(__synthesized_protocol, SynthesizedProtocol,
   OnConcreteNominalType,
-  UserInaccessible | RejectByParser | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | RejectByParser | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnconstrainedInABIAttr,
   54)
 
 SIMPLE_DECL_ATTR(testable, Testable,
   OnImport,
-  UserInaccessible | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   55)
 
 DECL_ATTR(_alignment, Alignment,
   OnStruct | OnEnum,
-  UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   56)
 
 SIMPLE_DECL_ATTR(rethrows, Rethrows,
   OnFunc | OnConstructor,
-  DeclModifier | RejectByParser | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove,
+  DeclModifier | RejectByParser | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | UnconstrainedInABIAttr,
   57)
 
 SIMPLE_DECL_ATTR(rethrows, AtRethrows,
   OnProtocol,
-  ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove,
+  ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | UnreachableInABIAttr,
   58)
 
 DECL_ATTR(_swift_native_objc_runtime_base, SwiftNativeObjCRuntimeBase,
   OnClass,
-  UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   59)
 
 CONTEXTUAL_SIMPLE_DECL_ATTR(indirect, Indirect,
   OnEnum | OnEnumElement,
-  DeclModifier | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove,
+  DeclModifier | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   60)
 
 SIMPLE_DECL_ATTR(warn_unqualified_access, WarnUnqualifiedAccess,
   OnFunc | OnAccessor,
-  LongAttribute | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  LongAttribute | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   61)
 
 SIMPLE_DECL_ATTR(_show_in_interface, ShowInInterface,
   OnProtocol,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   62)
 
 DECL_ATTR(_cdecl, CDecl,
   OnFunc | OnAccessor,
-  LongAttribute | UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  LongAttribute | UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   63)
 
 SIMPLE_DECL_ATTR(usableFromInline, UsableFromInline,
   OnAbstractFunction | OnVar | OnSubscript | OnNominalType | OnTypeAlias,
-  LongAttribute | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  LongAttribute | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   64)
 
 SIMPLE_DECL_ATTR(discardableResult, DiscardableResult,
   OnFunc | OnAccessor | OnConstructor | OnMacro,
-  LongAttribute | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  LongAttribute | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   65)
 
 SIMPLE_DECL_ATTR(GKInspectable, GKInspectable,
   OnVar,
-  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   66)
 
 DECL_ATTR(_implements, Implements,
   OnFunc | OnAccessor | OnVar | OnSubscript | OnTypeAlias,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   67)
 
 DECL_ATTR(_objcRuntimeName, ObjCRuntimeName,
   OnClass,
-  UserInaccessible | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   68)
 
 SIMPLE_DECL_ATTR(_staticInitializeObjCMetadata, StaticInitializeObjCMetadata,
   OnClass,
-  UserInaccessible | LongAttribute | RejectByParser | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | LongAttribute | RejectByParser | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   69)
 
 DECL_ATTR(_restatedObjCConformance, RestatedObjCConformance,
   OnProtocol,
-  UserInaccessible | LongAttribute | RejectByParser | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | LongAttribute | RejectByParser | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnconstrainedInABIAttr,
   70)
 
 // Unused '71'
 
 DECL_ATTR(implementation, ObjCImplementation,
   OnExtension | OnAbstractFunction,
-  UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove,
+  UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | ForbiddenInABIAttr,
   72)
 DECL_ATTR_ALIAS(_objcImplementation, ObjCImplementation)
 
 DECL_ATTR(_optimize, Optimize,
   OnAbstractFunction | OnSubscript | OnVar,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   73)
 
 DECL_ATTR(_clangImporterSynthesizedType, ClangImporterSynthesizedType,
   OnGenericType,
-  LongAttribute | RejectByParser | UserInaccessible | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  LongAttribute | RejectByParser | UserInaccessible | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnconstrainedInABIAttr,
   74)
 
 SIMPLE_DECL_ATTR(_weakLinked, WeakLinked,
   OnNominalType | OnAssociatedType | OnFunc | OnAccessor | OnVar | OnSubscript | OnConstructor | OnEnumElement | OnExtension | OnImport,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   75)
 
 SIMPLE_DECL_ATTR(frozen, Frozen,
   OnEnum | OnStruct,
-  ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToRemove | APIStableToAdd,
+  ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToRemove | APIStableToAdd | UnreachableInABIAttr,
   76)
-
 DECL_ATTR_ALIAS(_frozen, Frozen)
+
 SIMPLE_DECL_ATTR(_forbidSerializingReference, ForbidSerializingReference,
   OnAnyDecl,
-  LongAttribute | RejectByParser | UserInaccessible | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  LongAttribute | RejectByParser | UserInaccessible | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnconstrainedInABIAttr,
   77)
 
 SIMPLE_DECL_ATTR(_hasInitialValue, HasInitialValue,
   OnVar,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnconstrainedInABIAttr,
   78)
 
 SIMPLE_DECL_ATTR(_nonoverride, NonOverride,
   OnFunc | OnAccessor | OnVar | OnSubscript | OnConstructor | OnAssociatedType,
-  UserInaccessible | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   79)
 
 DECL_ATTR(_dynamicReplacement, DynamicReplacement,
   OnAbstractFunction | OnVar | OnSubscript,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   80)
 
 SIMPLE_DECL_ATTR(_borrowed, Borrowed,
   OnVar | OnSubscript,
-  UserInaccessible | NotSerialized | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | NotSerialized | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove | InferredInABIAttr,
   81)
 
 DECL_ATTR(_private, PrivateImport,
   OnImport,
-  UserInaccessible | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   82)
 
 SIMPLE_DECL_ATTR(_alwaysEmitIntoClient, AlwaysEmitIntoClient,
   OnVar | OnSubscript | OnAbstractFunction,
-  UserInaccessible | ABIBreakingToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIBreakingToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | InferredInABIAttr,
   83)
 
 SIMPLE_DECL_ATTR(_implementationOnly, ImplementationOnly,
   OnImport | OnFunc | OnConstructor | OnVar | OnSubscript,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   84)
 
 DECL_ATTR(_custom, Custom,
   OnAnyDecl,
-  RejectByParser | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  RejectByParser | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnconstrainedInABIAttr,
   85)
 
 SIMPLE_DECL_ATTR(propertyWrapper, PropertyWrapper,
   OnStruct | OnClass | OnEnum,
-  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   86)
 
 SIMPLE_DECL_ATTR(_disfavoredOverload, DisfavoredOverload,
   OnAbstractFunction | OnVar | OnSubscript,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   87)
 
 SIMPLE_DECL_ATTR(resultBuilder, ResultBuilder,
   OnNominalType,
-  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   88)
 
 DECL_ATTR(_projectedValueProperty, ProjectedValueProperty,
   OnVar,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnconstrainedInABIAttr,
   89)
 
 SIMPLE_DECL_ATTR(_nonEphemeral, NonEphemeral,
   OnParam,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIBreakingToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIBreakingToAdd | APIStableToRemove | ForbiddenInABIAttr,
   90)
 
 DECL_ATTR(differentiable, Differentiable,
   OnAccessor | OnConstructor | OnFunc | OnVar | OnSubscript,
-  LongAttribute | AllowMultipleAttributes | ABIStableToAdd | ABIBreakingToRemove | APIStableToAdd | APIBreakingToRemove,
+  LongAttribute | AllowMultipleAttributes | ABIStableToAdd | ABIBreakingToRemove | APIStableToAdd | APIBreakingToRemove | ForbiddenInABIAttr,
   91)
 
 SIMPLE_DECL_ATTR(_hasMissingDesignatedInitializers, HasMissingDesignatedInitializers,
   OnClass,
-  UserInaccessible | NotSerialized | APIBreakingToAdd | ABIBreakingToAdd | APIStableToRemove | ABIStableToRemove,
+  UserInaccessible | NotSerialized | APIBreakingToAdd | ABIBreakingToAdd | APIStableToRemove | ABIStableToRemove | UnreachableInABIAttr,
   92)
 
 SIMPLE_DECL_ATTR(_inheritsConvenienceInitializers, InheritsConvenienceInitializers,
   OnClass,
-  UserInaccessible | NotSerialized | APIStableToAdd | ABIStableToAdd | APIBreakingToRemove | ABIBreakingToRemove,
+  UserInaccessible | NotSerialized | APIStableToAdd | ABIStableToAdd | APIBreakingToRemove | ABIBreakingToRemove | UnreachableInABIAttr,
   93)
 
 DECL_ATTR(_typeEraser, TypeEraser,
   OnProtocol,
-  UserInaccessible | ABIStableToAdd | ABIBreakingToRemove | APIStableToAdd | APIBreakingToRemove | AllowMultipleAttributes,
+  UserInaccessible | ABIStableToAdd | ABIBreakingToRemove | APIStableToAdd | APIBreakingToRemove | AllowMultipleAttributes | UnreachableInABIAttr,
   94)
 
 SIMPLE_DECL_ATTR(IBSegueAction, IBSegueAction,
   OnFunc,
-  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   95)
 
 DECL_ATTR(_originallyDefinedIn, OriginallyDefinedIn,
   OnNominalType | OnFunc | OnVar | OnExtension,
-  UserInaccessible | AllowMultipleAttributes | LongAttribute | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | AllowMultipleAttributes | LongAttribute | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove | UnconstrainedInABIAttr,
   96)
 
 DECL_ATTR(derivative, Derivative,
   OnFunc,
-  LongAttribute | AllowMultipleAttributes | ABIStableToAdd | ABIBreakingToRemove | APIStableToAdd | APIBreakingToRemove,
+  LongAttribute | AllowMultipleAttributes | ABIStableToAdd | ABIBreakingToRemove | APIStableToAdd | APIBreakingToRemove | ForbiddenInABIAttr,
   97)
 
 DECL_ATTR(_spi, SPIAccessControl,
   OnAbstractFunction | OnExtension | OnGenericType | OnVar | OnSubscript | OnImport | OnAccessor | OnEnumElement | OnMacro,
-  AllowMultipleAttributes | UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIBreakingToAdd | APIStableToRemove,
+  AllowMultipleAttributes | UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIBreakingToAdd | APIStableToRemove | ForbiddenInABIAttr,
   98)
 
 DECL_ATTR(transpose, Transpose,
   OnFunc,
-  LongAttribute | AllowMultipleAttributes | ABIStableToAdd | ABIBreakingToRemove | APIStableToAdd | APIBreakingToRemove,
+  LongAttribute | AllowMultipleAttributes | ABIStableToAdd | ABIBreakingToRemove | APIStableToAdd | APIBreakingToRemove | ForbiddenInABIAttr,
   99)
 
 SIMPLE_DECL_ATTR(noDerivative, NoDerivative,
   OnAbstractFunction | OnVar | OnSubscript,
-  ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove,
+  ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | ForbiddenInABIAttr,
   100)
 
 // Unused '101'
 
 CONTEXTUAL_SIMPLE_DECL_ATTR(actor, Actor,
   OnClass,
-  DeclModifier | ConcurrencyOnly | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove,
+  DeclModifier | ConcurrencyOnly | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | UnreachableInABIAttr,
   102)
 
 CONTEXTUAL_SIMPLE_DECL_ATTR(isolated, Isolated,
   OnDestructor,
-  DeclModifier | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove,
+  DeclModifier | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | UnreachableInABIAttr,
   103)
 
 SIMPLE_DECL_ATTR(globalActor, GlobalActor,
   OnClass | OnStruct | OnEnum,
-  ABIStableToAdd | ABIBreakingToRemove | APIStableToAdd | APIBreakingToRemove,
+  ABIStableToAdd | ABIBreakingToRemove | APIStableToAdd | APIBreakingToRemove | UnreachableInABIAttr,
   104)
 
 SIMPLE_DECL_ATTR(_specializeExtension, SpecializeExtension,
   OnExtension,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   105)
 
 CONTEXTUAL_SIMPLE_DECL_ATTR(async, Async,
   OnVar | OnFunc,
-  DeclModifier | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove,
+  DeclModifier | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | UnconstrainedInABIAttr,
   106)
 
 SIMPLE_DECL_ATTR(Sendable, Sendable,
   OnFunc | OnConstructor | OnAccessor | OnAnyClangDecl,
-  ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove,
+  ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | UnconstrainedInABIAttr,
   107)
+
 SIMPLE_DECL_ATTR(_marker, Marker,
   OnProtocol,
-  UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove,
+  UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | UnreachableInABIAttr,
   108)
 
 SIMPLE_DECL_ATTR(reasync, Reasync,
   OnFunc | OnConstructor,
-  RejectByParser | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove,
+  RejectByParser | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | UnconstrainedInABIAttr,
   109)
 
 SIMPLE_DECL_ATTR(reasync, AtReasync,
   OnProtocol,
-  ConcurrencyOnly | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove,
+  ConcurrencyOnly | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | UnreachableInABIAttr,
   110)
 
 // Unused '111'
 
 CONTEXTUAL_DECL_ATTR(nonisolated, Nonisolated,
   OnFunc | OnConstructor | OnDestructor | OnVar | OnSubscript | OnProtocol | OnExtension | OnClass | OnStruct | OnEnum,
-  DeclModifier | ABIStableToAdd | ABIStableToRemove | APIBreakingToAdd | APIStableToRemove,
+  DeclModifier | ABIStableToAdd | ABIStableToRemove | APIBreakingToAdd | APIStableToRemove | UnconstrainedInABIAttr,
   112)
 
 // Unused '113'
 
 SIMPLE_DECL_ATTR(_unsafeInheritExecutor, UnsafeInheritExecutor,
   OnFunc,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIBreakingToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIBreakingToRemove | UnconstrainedInABIAttr,
   114)
 
 SIMPLE_DECL_ATTR(_implicitSelfCapture, ImplicitSelfCapture,
   OnParam,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIBreakingToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIBreakingToRemove | ForbiddenInABIAttr,
   115)
 
 SIMPLE_DECL_ATTR(_inheritActorContext, InheritActorContext,
   OnParam,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIBreakingToAdd | APIBreakingToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIBreakingToAdd | APIBreakingToRemove | ForbiddenInABIAttr,
   116)
 
 SIMPLE_DECL_ATTR(_eagerMove, EagerMove,
   OnFunc | OnParam | OnVar | OnNominalType,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   117)
 
 CONTEXTUAL_SIMPLE_DECL_ATTR(distributed, DistributedActor,
   OnClass | OnFunc | OnAccessor | OnVar,
-  DeclModifier | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove,
+  DeclModifier | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | EquivalentInABIAttr,
   118)
 
 SIMPLE_DECL_ATTR(_noEagerMove, NoEagerMove,
   OnFunc | OnParam | OnVar | OnNominalType,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   119)
 
 SIMPLE_DECL_ATTR(_assemblyVision, EmitAssemblyVisionRemarks,
   OnFunc | OnNominalType,
-  UserInaccessible | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   120)
 
 DECL_ATTR(_nonSendable, NonSendable,
   OnNominalType,
-  UserInaccessible | AllowMultipleAttributes | ABIStableToAdd | ABIBreakingToRemove | APIStableToAdd | APIBreakingToRemove,
+  UserInaccessible | AllowMultipleAttributes | ABIStableToAdd | ABIBreakingToRemove | APIStableToAdd | APIBreakingToRemove | UnconstrainedInABIAttr,
   121)
 
 SIMPLE_DECL_ATTR(_noImplicitCopy, NoImplicitCopy,
   OnFunc | OnParam | OnVar,
-  UserInaccessible | ABIStableToAdd | ABIBreakingToRemove | APIStableToAdd | APIBreakingToRemove,
+  UserInaccessible | ABIStableToAdd | ABIBreakingToRemove | APIStableToAdd | APIBreakingToRemove | ForbiddenInABIAttr,
   122)
 
 SIMPLE_DECL_ATTR(_noLocks, NoLocks,
   OnAbstractFunction | OnSubscript,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   123)
 
 SIMPLE_DECL_ATTR(_noAllocation, NoAllocation,
   OnAbstractFunction | OnSubscript,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   124)
 
 SIMPLE_DECL_ATTR(preconcurrency, Preconcurrency,
   OnFunc | OnConstructor | OnProtocol | OnGenericType | OnVar | OnSubscript | OnEnumElement | OnImport,
-  ABIStableToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove,
+  ABIStableToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | UnconstrainedInABIAttr,
   125)
 
 CONTEXTUAL_SIMPLE_DECL_ATTR(_const, CompileTimeLiteral,
   OnParam | OnVar,
-  DeclModifier | UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIBreakingToAdd | APIStableToRemove,
+  DeclModifier | UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIBreakingToAdd | APIStableToRemove | UnconstrainedInABIAttr,
   126)
 
 DECL_ATTR(_unavailableFromAsync, UnavailableFromAsync,
   OnFunc | OnConstructor | OnMacro,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIBreakingToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIBreakingToAdd | APIStableToRemove | ForbiddenInABIAttr,
   127)
 
 DECL_ATTR(exclusivity, Exclusivity,
   OnVar,
-  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   128)
 
 DECL_ATTR(backDeployed, BackDeployed,
   OnAbstractFunction | OnAccessor | OnSubscript | OnVar,
-  AllowMultipleAttributes | LongAttribute | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIBreakingToRemove,
+  AllowMultipleAttributes | LongAttribute | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIBreakingToRemove | ForbiddenInABIAttr,
   129)
 DECL_ATTR_ALIAS(_backDeploy, BackDeployed)
 
 CONTEXTUAL_SIMPLE_DECL_ATTR(_local, KnownToBeLocal,
   OnFunc | OnParam | OnVar,
-  DeclModifier | UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove,
+  DeclModifier | UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | UnconstrainedInABIAttr,
   130)
 
 SIMPLE_DECL_ATTR(_moveOnly, MoveOnly,
   OnNominalType,
-  UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove,
+  UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | UnreachableInABIAttr,
   131)
 
 SIMPLE_DECL_ATTR(_alwaysEmitConformanceMetadata, AlwaysEmitConformanceMetadata,
   OnProtocol,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   132)
 
 DECL_ATTR(_expose, Expose,
   OnFunc | OnNominalType | OnVar | OnConstructor,
-  AllowMultipleAttributes | LongAttribute | UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  AllowMultipleAttributes | LongAttribute | UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   133)
 
 // Unused '134'
 
 SIMPLE_DECL_ATTR(_spiOnly, SPIOnly,
   OnImport,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   135)
 
 DECL_ATTR(_documentation, Documentation,
   OnAnyDecl,
-  UserInaccessible | APIBreakingToAdd | APIStableToRemove | ABIStableToAdd | ABIStableToRemove,
+  UserInaccessible | APIBreakingToAdd | APIStableToRemove | ABIStableToAdd | ABIStableToRemove | ForbiddenInABIAttr,
   136)
 
 // Unused '137'
 
 SIMPLE_DECL_ATTR(_noMetadata, NoMetadata,
   OnGenericTypeParam,
-  UserInaccessible | NotSerialized | ABIStableToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | NotSerialized | ABIStableToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   138)
 
 // Unused '139'
 
 CONTEXTUAL_SIMPLE_DECL_ATTR(consuming, Consuming,
   OnFunc | OnAccessor,
-  DeclModifier | NotSerialized | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove,
+  DeclModifier | NotSerialized | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove | UnconstrainedInABIAttr,
   140)
 
 CONTEXTUAL_SIMPLE_DECL_ATTR(borrowing, Borrowing,
   OnFunc | OnAccessor,
-  DeclModifier | NotSerialized | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove,
+  DeclModifier | NotSerialized | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove | UnconstrainedInABIAttr,
   141)
 
 DECL_ATTR(attached, MacroRole,
   OnMacro,
-  AllowMultipleAttributes | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIBreakingToRemove,
+  AllowMultipleAttributes | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIBreakingToRemove | UnreachableInABIAttr,
   142)
 DECL_ATTR_ALIAS(freestanding, MacroRole)
 
 SIMPLE_DECL_ATTR(_used, Used,
   OnAbstractFunction | OnVar,
-  UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove,
+  UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | ForbiddenInABIAttr,
   143)
 
 DECL_ATTR(_section, Section,
   OnAbstractFunction | OnVar,
-  UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove,
+  UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | ForbiddenInABIAttr,
   144)
 
 DECL_ATTR(storageRestrictions, StorageRestrictions,
   OnAccessor,
-  ABIStableToAdd | ABIStableToRemove | APIBreakingToAdd | APIBreakingToRemove,
+  ABIStableToAdd | ABIStableToRemove | APIBreakingToAdd | APIBreakingToRemove | UnreachableInABIAttr,
   145)
 
 DECL_ATTR(_rawLayout, RawLayout,
   OnStruct,
-  UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   146)
 
 DECL_ATTR(_extern, Extern,
   OnFunc,
-  AllowMultipleAttributes | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove,
+  AllowMultipleAttributes | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   147)
 
 SIMPLE_DECL_ATTR(_nonescapable, NonEscapable,
   OnNominalType,
-  UserInaccessible | ABIBreakingToAdd | ABIStableToRemove | APIBreakingToAdd | APIStableToRemove,
+  UserInaccessible | ABIBreakingToAdd | ABIStableToRemove | APIBreakingToAdd | APIStableToRemove | UnreachableInABIAttr,
   148)
 
 SIMPLE_DECL_ATTR(_unsafeNonescapableResult, UnsafeNonEscapableResult,
   OnAbstractFunction | OnSubscript | OnAccessor,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIBreakingToAdd | APIBreakingToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIBreakingToAdd | APIBreakingToRemove | EquivalentInABIAttr,
   149)
 
 // Unused '150'
 
 SIMPLE_DECL_ATTR(_staticExclusiveOnly, StaticExclusiveOnly,
   OnStruct,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIBreakingToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIBreakingToAdd | APIStableToRemove | UnreachableInABIAttr,
   151)
 
 SIMPLE_DECL_ATTR(extractConstantsFromMembers, ExtractConstantsFromMembers,
   OnClass | OnEnum | OnProtocol | OnStruct,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   152)
 
 SIMPLE_DECL_ATTR(_noRuntime, NoRuntime,
   OnAbstractFunction | OnSubscript,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   153)
 
 SIMPLE_DECL_ATTR(_noExistentials, NoExistentials,
   OnAbstractFunction | OnSubscript,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   154)
 
 SIMPLE_DECL_ATTR(_noObjCBridging, NoObjCBridging,
   OnAbstractFunction | OnSubscript,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   155)
 
 // Unused '156': Used to be `_distributedThunkTarget` but completed implementation in Swift 6.0 does not need it after all
 
 DECL_ATTR(_allowFeatureSuppression, AllowFeatureSuppression,
   OnAnyDecl,
-  UserInaccessible | NotSerialized | ABIStableToAdd | APIStableToAdd | ABIStableToRemove | APIStableToRemove,
+  UserInaccessible | NotSerialized | ABIStableToAdd | APIStableToAdd | ABIStableToRemove | APIStableToRemove | ForbiddenInABIAttr,
   157)
 DECL_ATTR_ALIAS(_disallowFeatureSuppression, AllowFeatureSuppression)
 
 SIMPLE_DECL_ATTR(_preInverseGenerics, PreInverseGenerics,
   OnAbstractFunction | OnSubscript | OnVar | OnExtension,
-  UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove | UnconstrainedInABIAttr,
   158)
 
 // Declares that a struct contains "sensitive" data. It enforces that the contents of such a struct value
@@ -827,54 +828,54 @@ SIMPLE_DECL_ATTR(_preInverseGenerics, PreInverseGenerics,
 // TODO: enable @sensitive also for other nominal types than structs, e.g. for enums
 SIMPLE_DECL_ATTR(sensitive, Sensitive,
   OnStruct,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIBreakingToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIBreakingToAdd | APIStableToRemove | UnreachableInABIAttr,
   159)
 
 SIMPLE_DECL_ATTR(unsafe, Unsafe,
   OnAbstractFunction | OnSubscript | OnVar | OnMacro | OnNominalType | OnExtension | OnTypeAlias | OnEnumElement,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   160)
 
 DECL_ATTR(lifetime, Lifetime,
   OnAccessor | OnConstructor | OnFunc | OnSubscript,
-  LongAttribute | ABIBreakingToAdd | ABIStableToRemove | APIBreakingToAdd | APIStableToRemove | AllowMultipleAttributes,
+  LongAttribute | ABIBreakingToAdd | ABIStableToRemove | APIBreakingToAdd | APIStableToRemove | AllowMultipleAttributes | EquivalentInABIAttr,
   161)
 
 SIMPLE_DECL_ATTR(_addressableSelf, AddressableSelf,
   OnAccessor | OnConstructor | OnFunc | OnSubscript,
-  ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | UserInaccessible,
+  ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | UserInaccessible | UnconstrainedInABIAttr,
   162)
 
 SIMPLE_DECL_ATTR(_addressableForDependencies, AddressableForDependencies,
   OnNominalType,
-  ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | UserInaccessible,
+  ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | UserInaccessible | UnreachableInABIAttr,
   163)
 
 SIMPLE_DECL_ATTR(safe, Safe,
   OnAbstractFunction | OnSubscript | OnVar | OnMacro | OnNominalType | OnExtension | OnEnumElement,
-  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   164)
 
 DECL_ATTR(abi, ABI,
-  OnAbstractFunction | OnVar,
-  LongAttribute | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  OnConstructor | OnFunc | OnSubscript | OnVar,
+  LongAttribute | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   165)
 DECL_ATTR_FEATURE_REQUIREMENT(ABI, ABIAttribute)
 
 DECL_ATTR(execution, Execution,
   OnFunc | OnConstructor | OnSubscript | OnVar,
-  ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove,
+  ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | UnconstrainedInABIAttr,
   166)
 DECL_ATTR_FEATURE_REQUIREMENT(Execution, ExecutionAttribute)
 
 SIMPLE_DECL_ATTR(const, ConstVal,
   OnParam | OnVar | OnFunc,
-  ABIStableToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove,
+  ABIStableToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | EquivalentInABIAttr,
   167)
 DECL_ATTR_FEATURE_REQUIREMENT(ConstVal, CompileTimeValues)
 SIMPLE_DECL_ATTR(constInitialized, ConstInitialized,
   OnVar,
-  ABIStableToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove,
+  ABIStableToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | EquivalentInABIAttr,
   168)
 DECL_ATTR_FEATURE_REQUIREMENT(ConstInitialized, CompileTimeValues)
 
diff --git a/include/swift/AST/DiagnosticEngine.h b/include/swift/AST/DiagnosticEngine.h
index 6a4d7c2cb4262..147f8a5c1f751 100644
--- a/include/swift/AST/DiagnosticEngine.h
+++ b/include/swift/AST/DiagnosticEngine.h
@@ -702,6 +702,10 @@ namespace swift {
     /// Flush the active diagnostic to the diagnostic output engine.
     void flush();
 
+    /// Returns the \c SourceManager associated with \c SourceLoc s for this
+    /// diagnostic.
+    SourceManager &getSourceManager();
+
     /// Prevent the diagnostic from behaving more severely than \p limit. For
     /// instance, if \c DiagnosticBehavior::Warning is passed, an error will be
     /// emitted as a warning, but a note will still be emitted as a note.
@@ -1561,6 +1565,10 @@ namespace swift {
     }
   };
 
+  inline SourceManager &InFlightDiagnostic::getSourceManager() {
+    return Engine->SourceMgr;
+  }
+
   /// Remember details about the state of a diagnostic engine and restore them
   /// when the object is destroyed.
   ///
diff --git a/include/swift/AST/DiagnosticsSema.def b/include/swift/AST/DiagnosticsSema.def
index 47cb892f60e7d..619d8d4ce78c4 100644
--- a/include/swift/AST/DiagnosticsSema.def
+++ b/include/swift/AST/DiagnosticsSema.def
@@ -8351,16 +8351,16 @@ ERROR(attr_abi_mismatched_kind,none,
 
 ERROR(attr_abi_mismatched_arity,none,
       "cannot give %kind0 the ABI of a %kindonly0 with a different number of "
-      "low-level parameters",
-      (ValueDecl *))
+      "%select{|generic }1parameters",
+      (Decl *, /*genericParams=*/bool))
 
 ERROR(attr_abi_mismatched_throws,none,
       "cannot give %0 the ABI of a %kindonly0 which %select{cannot|can}1 throw",
-      (ValueDecl *, /*abiCanThrow=*/bool))
+      (Decl *, /*abiCanThrow=*/bool))
 
 ERROR(attr_abi_mismatched_async,none,
       "cannot give %0 the ABI of %select{a non-async|an async}1 %kindonly0",
-      (ValueDecl *, /*abiIsAsync=*/bool))
+      (Decl *, /*abiIsAsync=*/bool))
 
 ERROR(attr_abi_mismatched_pbd_size,none,
       "cannot give pattern binding the ABI of a binding with "
@@ -8369,13 +8369,89 @@ ERROR(attr_abi_mismatched_pbd_size,none,
 
 ERROR(attr_abi_mismatched_var,none,
       "no match for %select{%kind0 in the ABI|ABI %kind0}1",
-      (ValueDecl *, /*isABI=*/bool))
+      (Decl *, /*isABI=*/bool))
 
 ERROR(attr_abi_incompatible_with_silgen_name,none,
       "cannot use '@_silgen_name' and '@abi' on the same %0 because they serve "
       "the same purpose",
       (DescriptiveDeclKind))
 
+ERROR(attr_abi_missing_attr,none,
+      "missing '%0' %select{attribute|modifier}1 in '@abi'",
+      (StringRef, bool))
+ERROR(attr_abi_extra_attr,none,
+      "extra %select{|implicit }2'%0' %select{attribute|modifier}1 in '@abi'",
+      (StringRef, bool, /*isImplicit=*/bool))
+ERROR(attr_abi_forbidden_attr,none,
+      "unused '%0' %select{attribute|modifier}1 in '@abi'",
+      (StringRef, bool))
+REMARK(abi_attr_inferred_attribute,none,
+       "inferred '%0' in '@abi' to match %select{attribute|modifier}1 on API",
+       (StringRef, bool))
+
+ERROR(attr_abi_mismatched_attr,none,
+      "'%0' %select{attribute|modifier}1 in '@abi' should match '%2'",
+      (StringRef, bool, StringRef))
+NOTE(attr_abi_matching_attr_here,none,
+     "%select{should match|matches}0 %select{attribute|modifier}1 "
+     "%select{|implicitly added }2here",
+     (/*matches=*/bool, /*isModifier=*/bool, /*isImplicit=*/bool))
+
+#define TYPE_ORIGIN(KIND_IDX, DECL_IDX) "%select{|%kind" #DECL_IDX " |" \
+                            "self parameter |result |thrown |%error}" #KIND_IDX
+ERROR(attr_abi_mismatched_type,none,
+      TYPE_ORIGIN(0, 1) "type %2 in '@abi' should match %3",
+      (unsigned, Decl *, Type, Type))
+NOTE(attr_abi_should_match_type_here,none,
+     "should match type here", ())
+
+ERROR(attr_abi_mismatched_generic_signature,none,
+      "generic signature '%0' in '@abi' is not compatible with '%1'",
+      (StringRef, StringRef))
+ERROR(attr_abi_missing_generic_signature,none,
+      "declaration in '@abi' should have generic signature compatible with "
+      "'%0'",
+      (StringRef))
+ERROR(attr_abi_extra_generic_signature,none,
+      "declaration in '@abi' should not have generic signature because %0 "
+      "is not generic",
+      (Decl *))
+
+ERROR(attr_abi_mismatched_param_modifier,none,
+      "%select{default |}0%select{attribute|modifier}2 %select{|'%0' }0"
+      "on " TYPE_ORIGIN(3, 4) "in '@abi' is not compatible with "
+      "%select{default|'%1'}1",
+      (StringRef, StringRef, /*isModifier=*/bool, unsigned, Decl *))
+ERROR(attr_abi_no_default_arguments,none,
+      "%kind0 in '@abi' should not have a default argument; it does not "
+      "affect the parameter's ABI",
+      (Decl *))
+
+// These macros insert 'final', 'non-final', or nothing depending on both the
+// current decl and its counterpart, such that 'non-final' is used if the
+// counterpart would be described as 'final' or 'static'. They must be kept in
+// sync with `StaticnessAndFinality`.
+#define NONFINAL_OR_NOTHING(COUNTERPART) \
+      "%select{||non-final |non-final |non-final |%error}" #COUNTERPART
+#define FINAL_OR_NONFINAL_OR_NOTHING(CURRENT, COUNTERPART, FINAL_OK) \
+      "%select{|%select{" NONFINAL_OR_NOTHING(COUNTERPART) \
+                      "|" NONFINAL_OR_NOTHING(COUNTERPART) \
+                      "|final |final ||%error}" #CURRENT "}" #FINAL_OK
+
+ERROR(attr_abi_static_final_mismatch,none,
+      FINAL_OR_NONFINAL_OR_NOTHING(0, 2, 4) "%kind1 in '@abi' should be "
+      FINAL_OR_NONFINAL_OR_NOTHING(2, 0, 4) "%kindonly3 to ensure ABI "
+      "compatibility",
+      (uint8_t, Decl *, uint8_t, Decl *, /*isClass=*/bool))
+
+#undef NONFINAL_OR_NOTHING
+#undef FINAL_OR_NONFINAL_OR_NOTHING
+
+ERROR(attr_abi_failable_mismatch,none,
+      "cannot give %select{non-failable|failable}1 %kind0 the ABI of a "
+      "%select{non-failable|failable}2 %kindonly0",
+      (Decl *, bool, bool))
+
 //===----------------------------------------------------------------------===//
 // MARK: Isolated conformances
 //===----------------------------------------------------------------------===//
diff --git a/include/swift/AST/Types.h b/include/swift/AST/Types.h
index 300107268c571..73dbd7a9fdd9b 100644
--- a/include/swift/AST/Types.h
+++ b/include/swift/AST/Types.h
@@ -2378,6 +2378,10 @@ enum class ParamSpecifier : uint8_t {
 
 StringRef getNameForParamSpecifier(ParamSpecifier name);
 
+/// What does \c ParamSpecifier::Default mean for a parameter that's directly
+/// attached to \p VD ? Pass \c nullptr for the value for a closure.
+ParamSpecifier getDefaultParamSpecifier(const ValueDecl *VD);
+
 /// Provide parameter type relevant flags, i.e. variadic, autoclosure, and
 /// escaping.
 class ParameterTypeFlags {
diff --git a/include/swift/Basic/LangOptions.h b/include/swift/Basic/LangOptions.h
index 5d9cac773637b..8a73b09290079 100644
--- a/include/swift/Basic/LangOptions.h
+++ b/include/swift/Basic/LangOptions.h
@@ -269,6 +269,9 @@ namespace swift {
     /// Emit a remark on early exit in explicit interface build
     bool EnableSkipExplicitInterfaceModuleBuildRemarks = false;
 
+    /// Emit a remark when \c \@abi infers an attribute or modifier.
+    bool EnableABIInferenceRemarks = false;
+
     ///
     /// Support for alternate usage modes
     ///
diff --git a/include/swift/Option/Options.td b/include/swift/Option/Options.td
index 5b85213f00ed6..73f76baea1f3f 100644
--- a/include/swift/Option/Options.td
+++ b/include/swift/Option/Options.td
@@ -468,6 +468,10 @@ def remark_module_serialization : Flag<["-"], "Rmodule-serialization">,
   Flags<[FrontendOption, DoesNotAffectIncrementalBuild]>,
   HelpText<"Emit remarks about module serialization">;
 
+def remark_abi_inference : Flag<["-"], "Rabi-inference">,
+  Flags<[FrontendOption, DoesNotAffectIncrementalBuild]>,
+  HelpText<"Emit a remark when an '@abi' attribute adds an attribute or modifier to the ABI declaration based on its presence in the API">;
+
 def emit_tbd : Flag<["-"], "emit-tbd">,
   HelpText<"Emit a TBD file">,
   Flags<[FrontendOption, NoInteractiveOption, SupplementaryOutput]>;
diff --git a/lib/AST/ASTMangler.cpp b/lib/AST/ASTMangler.cpp
index de62f634dd4f3..7af62c05439c4 100644
--- a/lib/AST/ASTMangler.cpp
+++ b/lib/AST/ASTMangler.cpp
@@ -1193,11 +1193,9 @@ getOverriddenSwiftProtocolObjCName(const ValueDecl *decl,
     return std::nullopt;
 
   // If there is an 'objc' attribute with a name, use that name.
-  if (auto objc = proto->getAttrs().getAttribute<ObjCAttr>()) {
-    if (auto name = objc->getName()) {
-      llvm::SmallString<4> buffer;
-      return std::string(name->getString(buffer));
-    }
+  if (auto objcName = proto->getExplicitObjCName()) {
+    llvm::SmallString<4> buffer;
+    return std::string(objcName->getString(buffer));
   }
 
   return std::nullopt;
@@ -3379,8 +3377,7 @@ void ASTMangler::appendFunctionSignature(AnyFunctionType *fn,
   }
 }
 
-static ParamSpecifier
-getDefaultOwnership(const ValueDecl *forDecl) {
+ParamSpecifier swift::getDefaultParamSpecifier(const ValueDecl *forDecl) {
   // `consuming` is the default ownership for initializers and setters.
   // Everything else defaults to borrowing.
   if (!forDecl) {
@@ -3451,7 +3448,7 @@ getParameterFlagsForMangling(ParameterTypeFlags flags,
 void ASTMangler::appendFunctionInputType(
     AnyFunctionType *fnType, ArrayRef<AnyFunctionType::Param> params,
     GenericSignature sig, const ValueDecl *forDecl, bool isRecursedInto) {
-  auto defaultSpecifier = getDefaultOwnership(forDecl);
+  auto defaultSpecifier = getDefaultParamSpecifier(forDecl);
   
   switch (params.size()) {
   case 0:
diff --git a/lib/AST/ASTPrinter.cpp b/lib/AST/ASTPrinter.cpp
index a96e0783c5381..7beec725f5709 100644
--- a/lib/AST/ASTPrinter.cpp
+++ b/lib/AST/ASTPrinter.cpp
@@ -1284,6 +1284,15 @@ void PrintAST::printAttributes(const Decl *D) {
   // for each decl They cannot be shared across different decls.
   assert(Options.ExcludeCustomAttrList.empty());
 
+  // If there is an `@abi` attr, we need to print it first so that it isn't
+  // affected by subsequent mutation of `Options.ExcludeAttrList`.
+  if (auto abiAttr = attrs.getAttribute<ABIAttr>()) {
+    if (Options.PrintImplicitAttrs && !Options.excludeAttr(abiAttr)) {
+      abiAttr->print(Printer, Options, D);
+      Options.ExcludeAttrList.push_back(DeclAttrKind::ABI);
+    }
+  }
+
   if (Options.PrintSyntheticSILGenName
         && !D->getAttrs().hasAttribute<SILGenNameAttr>()) {
     if (canPrintSyntheticSILGenName(D)) {
@@ -1334,7 +1343,8 @@ void PrintAST::printAttributes(const Decl *D) {
     // Add SPIs to both private and package interfaces
     if (!Options.printPublicInterface() &&
         DeclAttribute::canAttributeAppearOnDeclKind(
-            DeclAttrKind::SPIAccessControl, D->getKind())) {
+            DeclAttrKind::SPIAccessControl, D->getKind()) &&
+        !Options.excludeAttrKind(DeclAttrKind::SPIAccessControl)) {
       interleave(D->getSPIGroups(),
              [&](Identifier spiName) {
                Printer.printAttrName("_spi", true);
@@ -1358,7 +1368,7 @@ void PrintAST::printAttributes(const Decl *D) {
     // If the declaration is implicitly @objc, print the attribute now.
     if (auto VD = dyn_cast<ValueDecl>(D)) {
       if (VD->isObjC() && !isa<EnumElementDecl>(VD) &&
-          !attrs.hasAttribute<ObjCAttr>()) {
+          !attrs.hasAttribute<ObjCAttr>() && ABIRoleInfo(D).providesAPI()) {
         Printer.printAttrName("@objc");
         Printer << " ";
       }
diff --git a/lib/AST/AccessRequests.cpp b/lib/AST/AccessRequests.cpp
index c85aa726de7dc..996677cb07d01 100644
--- a/lib/AST/AccessRequests.cpp
+++ b/lib/AST/AccessRequests.cpp
@@ -43,6 +43,11 @@ AccessLevel
 AccessLevelRequest::evaluate(Evaluator &evaluator, ValueDecl *D) const {
   assert(!D->hasAccess());
 
+  // ABI decls share the access level of their API decl.
+  auto abiRole = ABIRoleInfo(D);
+  if (!abiRole.providesAPI() && abiRole.getCounterpart())
+    return abiRole.getCounterpart()->getFormalAccess();
+
   // Check if the decl has an explicit access control attribute.
   if (auto *AA = D->getAttrs().getAttribute<AccessControlAttr>())
     return AA->getAccess();
@@ -201,6 +206,12 @@ AccessLevel
 SetterAccessLevelRequest::evaluate(Evaluator &evaluator,
                                    AbstractStorageDecl *ASD) const {
   assert(!ASD->Accessors.getInt().hasValue());
+
+  // ABI decls share the access level of their API decl.
+  auto abiRole = ABIRoleInfo(ASD);
+  if (!abiRole.providesAPI() && abiRole.getCounterpart())
+    return abiRole.getCounterpart()->getSetterFormalAccess();
+
   if (auto *SAA = ASD->getAttrs().getAttribute<SetterAccessAttr>())
     return SAA->getAccess();
 
diff --git a/lib/AST/Attr.cpp b/lib/AST/Attr.cpp
index c1713a6b6278d..61b992437d66c 100644
--- a/lib/AST/Attr.cpp
+++ b/lib/AST/Attr.cpp
@@ -34,6 +34,7 @@
 #include "swift/Basic/Defer.h"
 #include "swift/Basic/QuotedString.h"
 #include "swift/Strings.h"
+#include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/Support/ErrorHandling.h"
@@ -58,13 +59,24 @@ static_assert(IsTriviallyDestructible<DeclAttributes>::value,
                 #Name " needs to specify either APIBreakingToAdd or APIStableToAdd"); \
   static_assert(DeclAttribute::hasOneBehaviorFor##Id( \
                 DeclAttribute::APIBreakingToRemove | DeclAttribute::APIStableToRemove), \
-                #Name " needs to specify either APIBreakingToRemove or APIStableToRemove");
+                #Name " needs to specify either APIBreakingToRemove or APIStableToRemove"); \
+  static_assert(DeclAttribute::hasOneBehaviorFor##Id(DeclAttribute::InABIAttrMask), \
+                #Name " needs to specify exactly one of ForbiddenInABIAttr, UnconstrainedInABIAttr, EquivalentInABIAttr, InferredInABIAttr, or UnreachableInABIAttr");
 #include "swift/AST/DeclAttr.def"
 
 #define TYPE_ATTR(_, Id)                                                       \
   static_assert(TypeAttrKind::Id <= TypeAttrKind::Last_TypeAttr);
 #include "swift/AST/TypeAttr.def"
 
+LLVM_ATTRIBUTE_USED StringRef swift::getDeclAttrKindID(DeclAttrKind kind) {
+    switch (kind) {
+#define DECL_ATTR(_, CLASS, ...)                               \
+  case DeclAttrKind::CLASS:                                    \
+    return #CLASS;
+#include "swift/AST/DeclAttr.def"
+  }
+}
+
 StringRef swift::getAccessLevelSpelling(AccessLevel value) {
   switch (value) {
   case AccessLevel::Private: return "private";
@@ -392,6 +404,27 @@ bool DeclAttribute::canClone() const {
   }
 }
 
+// Ensure that every DeclAttribute subclass implements its own isEquivalent().
+static void checkDeclAttributeIsEquivalents() {
+#define DECL_ATTR(_,CLASS,...) \
+  bool(CLASS##Attr::*ptr##CLASS)(const CLASS##Attr *, Decl *) const = &CLASS##Attr::isEquivalent; \
+  (void)ptr##CLASS;
+#include "swift/AST/DeclAttr.def"
+}
+
+bool DeclAttribute::isEquivalent(const DeclAttribute *other, Decl *attachedTo) const {
+  (void)checkDeclAttributeIsEquivalents;
+  if (getKind() != other->getKind())
+    return false;
+  switch (getKind()) {
+#define DECL_ATTR(_,CLASS, ...) \
+  case DeclAttrKind::CLASS:\
+    return static_cast<const CLASS##Attr *>(this)->isEquivalent(\
+                static_cast<const CLASS##Attr *>(other), attachedTo);
+#include "swift/AST/DeclAttr.def"
+  }
+}
+
 const BackDeployedAttr *
 DeclAttributes::getBackDeployed(const ASTContext &ctx,
                                 bool forTargetVariant) const {
@@ -1664,8 +1697,26 @@ bool DeclAttribute::printImpl(ASTPrinter &Printer, const PrintOptions &Options,
       abiDecl = cast<PatternBindingDecl>(abiDecl)
                     ->getVarAtSimilarStructuralPosition(
                                       const_cast<VarDecl *>(cast<VarDecl>(D)));
-    if (abiDecl)
-      abiDecl->print(Printer, Options);
+    if (abiDecl) {
+      auto optionsCopy = Options;
+
+      // Don't print any attributes marked with `ForbiddenInABIAttr`.
+      // (Reminder: There is manual logic in `PrintAST::printAttributes()`
+      // to handle non-ABI attributes when `PrintImplicitAttrs` is set.)
+      for (auto rawAttrKind : range(0, unsigned(DeclAttrKind::Last_DeclAttr))) {
+        DeclAttrKind attrKind{rawAttrKind}; 
+        if (!(DeclAttribute::getBehaviors(attrKind)
+                & DeclAttribute::ForbiddenInABIAttr))
+          continue;
+
+        if (attrKind == DeclAttrKind::AccessControl)
+          optionsCopy.PrintAccess = false;
+        else
+          optionsCopy.ExcludeAttrList.push_back(attrKind);
+      }
+
+      abiDecl->print(Printer, optionsCopy);
+    }
     Printer << ")";
 
     break;
@@ -1730,7 +1781,7 @@ uint64_t DeclAttribute::getBehaviors(DeclAttrKind DK) {
     return BEHAVIORS;
 #include "swift/AST/DeclAttr.def"
   }
-  llvm_unreachable("bad DeclAttrKind");
+  return 0;
 }
 
 std::optional<Feature> DeclAttribute::getRequiredFeature(DeclAttrKind DK) {
@@ -2121,6 +2172,26 @@ Type TypeEraserAttr::getResolvedType(const ProtocolDecl *PD) const {
                            ErrorType::get(ctx));
 }
 
+static bool eqTypes(Type first, Type second) {
+  if (first.isNull() || second.isNull())
+    return false;
+  return first->getCanonicalType() == second->getCanonicalType();
+}
+
+bool TypeEraserAttr::isEquivalent(const TypeEraserAttr *other,
+                                  Decl *attachedTo) const {
+  // Only makes sense when attached to a protocol.
+  auto proto = dyn_cast<ProtocolDecl>(attachedTo);
+  if (!proto)
+    return true;
+
+  Type thisType = getResolvedType(proto),
+       otherType = other->getResolvedType(proto);
+  if (thisType.isNull() || otherType.isNull())
+    return false;
+  return thisType->getCanonicalType() == otherType->getCanonicalType();
+}
+
 Type RawLayoutAttr::getResolvedLikeType(StructDecl *sd) const {
   auto &ctx = sd->getASTContext();
   return evaluateOrDefault(ctx.evaluator,
@@ -2139,6 +2210,34 @@ Type RawLayoutAttr::getResolvedCountType(StructDecl *sd) const {
                            ErrorType::get(ctx));
 }
 
+bool RawLayoutAttr::isEquivalent(const RawLayoutAttr *other,
+                                 Decl *attachedTo) const {
+  if (shouldMoveAsLikeType() != other->shouldMoveAsLikeType())
+    return false;
+
+  if (auto thisSizeAndAlignment = getSizeAndAlignment())
+    return thisSizeAndAlignment == other->getSizeAndAlignment();
+
+  auto SD = dyn_cast<StructDecl>(attachedTo);
+  if (!SD)
+    return true;
+
+  if (auto thisScalarLikeType = getResolvedScalarLikeType(SD)) {
+    auto otherScalarLikeType = other->getResolvedScalarLikeType(SD);
+    return otherScalarLikeType && eqTypes(*thisScalarLikeType,
+                                          *otherScalarLikeType);
+  }
+
+  if (auto thisArrayLikeTypes = getResolvedArrayLikeTypeAndCount(SD)) {
+    auto otherArrayLikeTypes = other->getResolvedArrayLikeTypeAndCount(SD);
+    return otherArrayLikeTypes
+            && eqTypes(thisArrayLikeTypes->first, otherArrayLikeTypes->first)
+            && eqTypes(thisArrayLikeTypes->second, otherArrayLikeTypes->second);
+  }
+
+  llvm_unreachable("unknown variant of RawLayoutAttr");
+}
+
 AvailableAttr::AvailableAttr(
     SourceLoc AtLoc, SourceRange Range,
     AvailabilityDomainOrIdentifier DomainOrIdentifier, SourceLoc DomainLoc,
@@ -2246,6 +2345,19 @@ AvailableAttr *AvailableAttr::clone(ASTContext &C, bool implicit) const {
       implicit ? SourceRange() : ObsoletedRange, implicit, isSPI());
 }
 
+bool AvailableAttr::isEquivalent(const AvailableAttr *other,
+                                 Decl *attachedTo) const {
+  return getRawIntroduced() == other->getRawIntroduced()
+          && getRawDeprecated() == other->getRawDeprecated()
+          && getRawObsoleted() == other->getRawObsoleted()
+          && getMessage() == other->getMessage()
+          && getRename() == other->getRename()
+          && isSPI() == other->isSPI()
+          && getKind() == other->getKind()
+          && attachedTo->getSemanticAvailableAttr(this)->getDomain()
+                ==  attachedTo->getSemanticAvailableAttr(other)->getDomain();
+}
+
 static StringRef getManglingModuleName(StringRef OriginalModuleName) {
   auto index = OriginalModuleName.find(";");
   return index == StringRef::npos
@@ -2448,6 +2560,69 @@ GenericSignature SpecializeAttr::getSpecializedSignature(
                            nullptr);
 }
 
+/// Checks whether \p first and \p second have the same elements according to
+/// \p eq , ignoring the order those elements are in but considering the number
+/// of repetitions.
+template<typename Element, typename Eq>
+bool sameElements(ArrayRef<Element> first, ArrayRef<Element> second, Eq eq) {
+  if (first.size() != second.size())
+    return false;
+
+  llvm::SmallSet<unsigned, 8> claimedSecondIndices;
+
+  for (auto firstElem : first) {
+    bool found = false;
+    for (auto i : indices(second)) {
+      if (!eq(firstElem, second[i]) || claimedSecondIndices.count(i))
+        continue;
+
+      found = true;
+      claimedSecondIndices.insert(i);
+      break;
+    }
+
+    if (!found)
+      return false;
+  }
+
+  return true;
+}
+
+/// Checks whether \p first and \p second have the same elements, ignoring the
+/// order those elements are in but considering the number of repetitions.
+template<typename Element>
+bool sameElements(ArrayRef<Element> first, ArrayRef<Element> second) {
+  return sameElements(first, second, std::equal_to<Element>());
+}
+
+bool SpecializeAttr::isEquivalent(const SpecializeAttr *other,
+                                  Decl *attachedTo) const {
+  if (isExported() != other->isExported() ||
+      getSpecializationKind() != other->getSpecializationKind() ||
+      getTargetFunctionName() != other->getTargetFunctionName() ||
+      specializedSignature.getCanonicalSignature() !=
+        other->specializedSignature.getCanonicalSignature())
+    return false;
+
+  if (!sameElements(getSPIGroups(), other->getSPIGroups()))
+    return false;
+
+  SmallVector<CanType, 8> thisTypeErasedParams, otherTypeErasedParams;
+  for (auto ty : getTypeErasedParams())
+    thisTypeErasedParams.push_back(ty->getCanonicalType());
+  for (auto ty : other->getTypeErasedParams())
+    otherTypeErasedParams.push_back(ty->getCanonicalType());
+
+  if (!sameElements(ArrayRef(thisTypeErasedParams),
+                    ArrayRef(otherTypeErasedParams)))
+    return false;
+
+  return sameElements(getAvailableAttrs(), other->getAvailableAttrs(),
+                      [=](AvailableAttr *thisAttr, AvailableAttr *otherAttr) {
+    return thisAttr->isEquivalent(otherAttr, attachedTo);
+  });
+}
+
 SPIAccessControlAttr::SPIAccessControlAttr(SourceLoc atLoc, SourceRange range,
                                            ArrayRef<Identifier> spiGroups)
     : DeclAttribute(DeclAttrKind::SPIAccessControl, atLoc, range,
@@ -2476,6 +2651,11 @@ SPIAccessControlAttr *SPIAccessControlAttr::clone(ASTContext &C,
   return attr;
 }
 
+bool SPIAccessControlAttr::isEquivalent(const SPIAccessControlAttr *other,
+                                        Decl *attachedTo) const {
+  return sameElements(getSPIGroups(), other->getSPIGroups());
+}
+
 DifferentiableAttr::DifferentiableAttr(bool implicit, SourceLoc atLoc,
                                        SourceRange baseRange,
                                        enum DifferentiabilityKind diffKind,
@@ -2712,11 +2892,18 @@ StorageRestrictionsAttr::create(
                                            /*implicit=*/false);
 }
 
-ImplementsAttr::ImplementsAttr(SourceLoc atLoc, SourceRange range,
-                               TypeRepr *TyR, DeclName MemberName,
-                               DeclNameLoc MemberNameLoc)
+bool StorageRestrictionsAttr::
+isEquivalent(const StorageRestrictionsAttr *other, Decl *attachedTo) const {
+  return sameElements(getAccessesNames(), other->getAccessesNames())
+           && sameElements(getInitializesNames(), other->getInitializesNames());
+}
+
+ImplementsAttr::
+ImplementsAttr(SourceLoc atLoc, SourceRange range,
+               llvm::PointerUnion<TypeRepr *, DeclContext *> TyROrDC,
+               DeclName MemberName, DeclNameLoc MemberNameLoc)
     : DeclAttribute(DeclAttrKind::Implements, atLoc, range, /*Implicit=*/false),
-      TyR(TyR), MemberName(MemberName), MemberNameLoc(MemberNameLoc) {}
+      TyROrDC(TyROrDC), MemberName(MemberName), MemberNameLoc(MemberNameLoc) {}
 
 ImplementsAttr *ImplementsAttr::create(ASTContext &Ctx, SourceLoc atLoc,
                                        SourceRange range,
@@ -2733,9 +2920,8 @@ ImplementsAttr *ImplementsAttr::create(DeclContext *DC,
                                        DeclName MemberName) {
   auto &ctx = DC->getASTContext();
   void *mem = ctx.Allocate(sizeof(ImplementsAttr), alignof(ImplementsAttr));
-  auto *attr = new (mem) ImplementsAttr(
-      SourceLoc(), SourceRange(), nullptr,
-      MemberName, DeclNameLoc());
+  auto *attr = new (mem) ImplementsAttr(SourceLoc(), SourceRange(), DC,
+                                        MemberName, DeclNameLoc());
   ctx.evaluator.cacheOutput(ImplementsAttrProtocolRequest{attr, DC},
                             std::move(Proto));
   return attr;
@@ -2754,6 +2940,13 @@ ImplementsAttr::getCachedProtocol(DeclContext *dc) const {
   return std::nullopt;
 }
 
+bool ImplementsAttr::isEquivalent(const ImplementsAttr *other,
+                                  Decl *attachedTo) const {
+  auto DC = attachedTo->getDeclContext();
+  return getMemberName() == other->getMemberName()
+          && getProtocol(DC) == other->getProtocol(DC);
+}
+
 CustomAttr::CustomAttr(SourceLoc atLoc, SourceRange range, TypeExpr *type,
                        CustomAttributeInitializer *initContext,
                        ArgumentList *argList, bool implicit)
@@ -2822,6 +3015,14 @@ bool CustomAttr::isArgUnsafe() const {
   return isArgUnsafeBit;
 }
 
+bool CustomAttr::isEquivalent(const CustomAttr *other, Decl *attachedTo) const {
+  // For the sake of both @abi checking and implementability, we're going to
+  // ignore expressions in the arguments and initializer.
+
+  return isArgUnsafe() == other->isArgUnsafe() && eqTypes(getType(),
+                                                          other->getType());
+}
+
 MacroRoleAttr::MacroRoleAttr(SourceLoc atLoc, SourceRange range,
                              MacroSyntax syntax, SourceLoc lParenLoc,
                              MacroRole role,
@@ -2949,12 +3150,27 @@ AllowFeatureSuppressionAttr *AllowFeatureSuppressionAttr::create(
       AllowFeatureSuppressionAttr(atLoc, range, implicit, inverted, features);
 }
 
+bool AllowFeatureSuppressionAttr::
+isEquivalent(const AllowFeatureSuppressionAttr *other, Decl *attachedTo) const {
+  if (getInverted() != other->getInverted())
+    return false;
+
+  return sameElements(getSuppressedFeatures(), other->getSuppressedFeatures());
+}
+
 LifetimeAttr *LifetimeAttr::create(ASTContext &context, SourceLoc atLoc,
                                    SourceRange baseRange, bool implicit,
                                    LifetimeEntry *entry) {
   return new (context) LifetimeAttr(atLoc, baseRange, implicit, entry);
 }
 
+bool LifetimeAttr::isEquivalent(const LifetimeAttr *other,
+                                Decl *attachedTo) const {
+  // FIXME: This is kind of cheesy.
+  return getLifetimeEntry()->getString()
+      == other->getLifetimeEntry()->getString();
+}
+
 void swift::simple_display(llvm::raw_ostream &out, const DeclAttribute *attr) {
   if (attr)
     attr->print(out);
diff --git a/lib/AST/Availability.cpp b/lib/AST/Availability.cpp
index 5aa08cc6f99b3..af8113a89050d 100644
--- a/lib/AST/Availability.cpp
+++ b/lib/AST/Availability.cpp
@@ -411,6 +411,11 @@ bool Decl::isAvailableAsSPI() const {
 
 SemanticAvailableAttributes
 Decl::getSemanticAvailableAttrs(bool includeInactive) const {
+  // A decl in an @abi gets its availability from the decl it's attached to.
+  auto abiRole = ABIRoleInfo(this);
+  if (!abiRole.providesAPI() && abiRole.getCounterpart())
+    return abiRole.getCounterpart()->getSemanticAvailableAttrs(includeInactive);
+
   return SemanticAvailableAttributes(getAttrs(), this, includeInactive);
 }
 
diff --git a/lib/AST/Decl.cpp b/lib/AST/Decl.cpp
index 7953bba24d572..61dc314cbeace 100644
--- a/lib/AST/Decl.cpp
+++ b/lib/AST/Decl.cpp
@@ -4156,6 +4156,10 @@ void ValueDecl::setIsObjC(bool value) {
 }
 
 Identifier ExtensionDecl::getObjCCategoryName() const {
+  auto abiRole = ABIRoleInfo(this);
+  if (!abiRole.providesAPI() && abiRole.getCounterpart())
+    return abiRole.getCounterpart()->getObjCCategoryName();
+
   // If there's an @objc attribute, it's authoritative. (ClangImporter
   // attaches one automatically.)
   if (auto objcAttr = getAttrs().getAttribute<ObjCAttr>(/*AllowInvalid*/true)) {
@@ -4409,6 +4413,10 @@ StringRef ValueDecl::getCDeclName() const {
       return clangDecl->getName();
   }
 
+  auto abiRole = ABIRoleInfo(this);
+  if (!abiRole.providesAPI() && abiRole.getCounterpart())
+    return abiRole.getCounterpart()->getCDeclName();
+
   // Handle explicit cdecl attributes.
   if (auto cdeclAttr = getAttrs().getAttribute<CDeclAttr>()) {
     return cdeclAttr->Name;
@@ -4445,6 +4453,18 @@ ValueDecl::getObjCRuntimeName(bool skipIsObjCResolution) const {
   return std::nullopt;
 }
 
+std::optional<ObjCSelector>
+Decl::getExplicitObjCName(bool allowInvalid) const {
+  auto abiRole = ABIRoleInfo(this);
+  if (!abiRole.providesAPI() && abiRole.getCounterpart())
+    return abiRole.getCounterpart()->getExplicitObjCName();
+
+  auto objcAttr = getAttrs().getAttribute<ObjCAttr>(allowInvalid);
+  if (objcAttr && !objcAttr->isNameImplicit())
+    return objcAttr->getName();
+  return std::nullopt;
+}
+
 bool ValueDecl::canInferObjCFromRequirement(ValueDecl *requirement) {
   // Only makes sense for a requirement of an @objc protocol.
   auto proto = cast<ProtocolDecl>(requirement->getDeclContext());
@@ -4457,9 +4477,8 @@ bool ValueDecl::canInferObjCFromRequirement(ValueDecl *requirement) {
 
   // If there is already an @objc attribute with an explicit name, we
   // can't infer a name (it's already there).
-  if (auto objcAttr = getAttrs().getAttribute<ObjCAttr>()) {
-    if (objcAttr->hasName() && !objcAttr->isNameImplicit())
-      return false;
+  if (getExplicitObjCName().has_value()) {
+    return false;
   }
 
   // If the nominal type doesn't conform to the protocol at all, we
@@ -4700,6 +4719,11 @@ SourceLoc Decl::getAttributeInsertionLoc(bool forModifier) const {
 bool ValueDecl::isUsableFromInline() const {
   assert(getFormalAccess() < AccessLevel::Public);
 
+  // ABI decls share the access level of their API decl.
+  auto abiRole = ABIRoleInfo(this);
+  if (!abiRole.providesAPI() && abiRole.getCounterpart())
+    return abiRole.getCounterpart()->isUsableFromInline();
+
   if (getAttrs().hasAttribute<UsableFromInlineAttr>() ||
       getAttrs().hasAttribute<AlwaysEmitIntoClientAttr>() ||
       getAttrs().hasAttribute<InlinableAttr>())
@@ -5134,6 +5158,10 @@ static bool checkAccessUsingAccessScopes(const DeclContext *useDC,
 static bool
 isObjCMemberImplementation(const ValueDecl *VD,
                            llvm::function_ref<AccessLevel()> getAccessLevel) {
+  auto abiRole = ABIRoleInfo(VD);
+  if (!abiRole.providesAPI() && abiRole.getCounterpart())
+    return isObjCMemberImplementation(abiRole.getCounterpart(), getAccessLevel);
+
   if (auto ED = dyn_cast<ExtensionDecl>(VD->getDeclContext()))
     if (ED->isObjCImplementation() && !isa<TypeDecl>(VD)) {
       auto attrDecl = isa<AccessorDecl>(VD)
@@ -6613,6 +6641,10 @@ static StringRef mangleObjCRuntimeName(const NominalTypeDecl *nominal,
 
 StringRef ClassDecl::getObjCRuntimeName(
                        llvm::SmallVectorImpl<char> &buffer) const {
+   auto abiRole = ABIRoleInfo(this);
+   if (!abiRole.providesAPI() && abiRole.getCounterpart())
+     return abiRole.getCounterpart()->getObjCRuntimeName(buffer);
+
   // If there is a Clang declaration, use it's runtime name.
   if (auto objcClass
         = dyn_cast_or_null<clang::ObjCInterfaceDecl>(getClangDecl()))
@@ -7121,6 +7153,10 @@ ProtocolDecl::getPrimaryAssociatedTypes() const {
 
 StringRef ProtocolDecl::getObjCRuntimeName(
                           llvm::SmallVectorImpl<char> &buffer) const {
+   auto abiRole = ABIRoleInfo(this);
+   if (!abiRole.providesAPI() && abiRole.getCounterpart())
+     return abiRole.getCounterpart()->getObjCRuntimeName(buffer);
+
   // If there is an 'objc' attribute with a name, use that name.
   if (auto objc = getAttrs().getAttribute<ObjCAttr>()) {
     if (auto name = objc->getName())
@@ -7593,6 +7629,10 @@ getNameFromObjcAttribute(const ObjCAttr *attr, DeclName preferredName) {
 
 ObjCSelector
 AbstractStorageDecl::getObjCGetterSelector(Identifier preferredName) const {
+   auto abiRole = ABIRoleInfo(this);
+   if (!abiRole.providesAPI() && abiRole.getCounterpart())
+     return abiRole.getCounterpart()->getObjCGetterSelector(preferredName);
+
   // If the getter has an @objc attribute with a name, use that.
   if (auto getter = getAccessor(AccessorKind::Get)) {
       if (auto name = getNameFromObjcAttribute(getter->getAttrs().
@@ -7623,6 +7663,10 @@ AbstractStorageDecl::getObjCGetterSelector(Identifier preferredName) const {
 
 ObjCSelector
 AbstractStorageDecl::getObjCSetterSelector(Identifier preferredName) const {
+   auto abiRole = ABIRoleInfo(this);
+   if (!abiRole.providesAPI() && abiRole.getCounterpart())
+     return abiRole.getCounterpart()->getObjCSetterSelector(preferredName);
+
   // If the setter has an @objc attribute with a name, use that.
   auto setter = getAccessor(AccessorKind::Set);
   auto objcAttr = setter ? setter->getAttrs().getAttribute<ObjCAttr>()
@@ -7874,14 +7918,19 @@ SourceRange VarDecl::getSourceRange() const {
   return getNameLoc();
 }
 
-SourceRange VarDecl::getTypeSourceRangeForDiagnostics() const {
+SourceRange AbstractStorageDecl::getTypeSourceRangeForDiagnostics() const {
+  // Subscripts always have an explicitly-written type.
+  if (auto *SD = dyn_cast<SubscriptDecl>(this))
+    return SD->getElementTypeSourceRange();
+
   // For a parameter, map back to its parameter to get the TypeLoc.
   if (auto *PD = dyn_cast<ParamDecl>(this)) {
     if (auto typeRepr = PD->getTypeRepr())
       return typeRepr->getSourceRange();
   }
-  
-  Pattern *Pat = getParentPattern();
+
+  auto *VD = cast<VarDecl>(this);
+  Pattern *Pat = VD->getParentPattern();
   if (!Pat || Pat->isImplicit())
     return SourceRange();
 
@@ -8605,6 +8654,10 @@ Type VarDecl::getPropertyWrapperInitValueInterfaceType() const {
 }
 
 Identifier VarDecl::getObjCPropertyName() const {
+   auto abiRole = ABIRoleInfo(this);
+   if (!abiRole.providesAPI() && abiRole.getCounterpart())
+     return abiRole.getCounterpart()->getObjCPropertyName();
+
   if (auto attr = getAttrs().getAttribute<ObjCAttr>()) {
     if (auto name = attr->getName())
       return name->getSelectorPieces()[0];
@@ -10209,6 +10262,11 @@ AbstractFunctionDecl::getBodyFingerprintIncludingLocalTypeMembers() const {
 ObjCSelector
 AbstractFunctionDecl::getObjCSelector(DeclName preferredName,
                                       bool skipIsObjCResolution) const {
+   auto abiRole = ABIRoleInfo(this);
+   if (!abiRole.providesAPI() && abiRole.getCounterpart())
+     return abiRole.getCounterpart()->getObjCSelector(preferredName,
+                                                      skipIsObjCResolution);
+
   // FIXME: Forces computation of the Objective-C selector.
   if (!skipIsObjCResolution)
     (void)isObjC();
@@ -11640,7 +11698,12 @@ bool ClassDecl::isNonDefaultExplicitDistributedActor(ModuleDecl *M,
 bool ClassDecl::isNativeNSObjectSubclass() const {
   // @objc actors implicitly inherit from NSObject.
   if (isActor()) {
-    if (getAttrs().hasAttribute<ObjCAttr>()) {
+    DeclAttributes attrs = getAttrs();
+    auto abiRole = ABIRoleInfo(this);
+    if (!abiRole.providesAPI() && abiRole.getCounterpart())
+      attrs = abiRole.getCounterpart()->getAttrs();
+
+    if (attrs.hasAttribute<ObjCAttr>()) {
       return true;
     }
     ClassDecl *superclass = getSuperclassDecl();
diff --git a/lib/AST/Module.cpp b/lib/AST/Module.cpp
index eba98b6bbaa5c..f8ed0c23c6850 100644
--- a/lib/AST/Module.cpp
+++ b/lib/AST/Module.cpp
@@ -3165,6 +3165,11 @@ SPIGroupsRequest::evaluate(Evaluator &evaluator, const Decl *decl) const {
   assert (isa<ValueDecl>(decl) ||
           isa<ExtensionDecl>(decl));
 
+  // ABI decls share the SPI groups of their API decl.
+  auto abiRole = ABIRoleInfo(decl);
+  if (!abiRole.providesAPI() && abiRole.getCounterpart())
+    return abiRole.getCounterpart()->getSPIGroups();
+
   // First, look for local attributes.
   llvm::SetVector<Identifier> spiGroups;
   for (auto attr : decl->getAttrs().getAttributes<SPIAccessControlAttr>())
diff --git a/lib/AST/NameLookup.cpp b/lib/AST/NameLookup.cpp
index d806ce0499ec2..77c1e16fe3fce 100644
--- a/lib/AST/NameLookup.cpp
+++ b/lib/AST/NameLookup.cpp
@@ -1546,7 +1546,7 @@ void MemberLookupTable::addMember(Decl *member) {
     A->getMemberName().addToLookupTable(Lookup, vd);
 
   auto abiRole = ABIRoleInfo(vd);
-  if (!abiRole.providesABI())
+  if (!abiRole.providesABI() && abiRole.getCounterpart())
     addMember(abiRole.getCounterpart());
 }
 
diff --git a/lib/AST/SwiftNameTranslation.cpp b/lib/AST/SwiftNameTranslation.cpp
index 934c4db467dee..f675c536dcac7 100644
--- a/lib/AST/SwiftNameTranslation.cpp
+++ b/lib/AST/SwiftNameTranslation.cpp
@@ -37,6 +37,10 @@ getNameForObjC(const ValueDecl *VD, CustomNamesOnly_t customNamesOnly) {
   assert(isa<ClassDecl>(VD) || isa<ProtocolDecl>(VD) || isa<StructDecl>(VD) ||
          isa<EnumDecl>(VD) || isa<EnumElementDecl>(VD) ||
          isa<TypeAliasDecl>(VD));
+  auto abiRole = ABIRoleInfo(VD);
+  if (!abiRole.providesAPI() && abiRole.getCounterpart())
+    return getNameForObjC(abiRole.getCounterpart(), customNamesOnly);
+
   if (auto objc = VD->getAttrs().getAttribute<ObjCAttr>()) {
     if (auto name = objc->getName()) {
       assert(name->getNumSelectorPieces() == 1);
@@ -63,6 +67,10 @@ getErrorDomainStringForObjC(const EnumDecl *ED) {
   // Should have already been diagnosed as diag::objc_enum_generic.
   assert(!ED->isGenericContext() && "Trying to bridge generic enum error to Obj-C");
 
+  auto abiRole = ABIRoleInfo(ED);
+  if (!abiRole.providesAPI() && abiRole.getCounterpart())
+    return getErrorDomainStringForObjC(abiRole.getCounterpart());
+
   SmallVector<const NominalTypeDecl *, 4> outerTypes;
   for (const NominalTypeDecl * D = ED;
        D != nullptr;
@@ -86,6 +94,11 @@ getErrorDomainStringForObjC(const EnumDecl *ED) {
 bool swift::objc_translation::
 printSwiftEnumElemNameInObjC(const EnumElementDecl *EL, llvm::raw_ostream &OS,
                              Identifier PreferredName) {
+  auto abiRole = ABIRoleInfo(EL);
+  if (!abiRole.providesAPI() && abiRole.getCounterpart())
+    return printSwiftEnumElemNameInObjC(abiRole.getCounterpart(), OS,
+                                        PreferredName);
+
   StringRef ElemName = getNameForObjC(EL, CustomNamesOnly);
   if (!ElemName.empty()) {
     OS << ElemName;
@@ -104,6 +117,10 @@ printSwiftEnumElemNameInObjC(const EnumElementDecl *EL, llvm::raw_ostream &OS,
 
 std::pair<Identifier, ObjCSelector> swift::objc_translation::
 getObjCNameForSwiftDecl(const ValueDecl *VD, DeclName PreferredName){
+  auto abiRole = ABIRoleInfo(VD);
+  if (!abiRole.providesAPI() && abiRole.getCounterpart())
+    return getObjCNameForSwiftDecl(abiRole.getCounterpart(), PreferredName);
+
   ASTContext &Ctx = VD->getASTContext();
   Identifier BaseName;
   if (PreferredName) {
@@ -157,6 +174,10 @@ isVisibleToObjC(const ValueDecl *VD, AccessLevel minRequiredAccess,
 StringRef
 swift::cxx_translation::getNameForCxx(const ValueDecl *VD,
                                       CustomNamesOnly_t customNamesOnly) {
+  auto abiRole = ABIRoleInfo(VD);
+  if (!abiRole.providesAPI() && abiRole.getCounterpart())
+    return getNameForCxx(abiRole.getCounterpart(), customNamesOnly);
+
   ASTContext& ctx = VD->getASTContext();
 
   for (auto *EA : VD->getAttrs().getAttributes<ExposeAttr>()) {
@@ -214,6 +235,10 @@ swift::cxx_translation::DeclRepresentation
 swift::cxx_translation::getDeclRepresentation(
     const ValueDecl *VD,
     std::optional<std::function<bool(const NominalTypeDecl *)>> isZeroSized) {
+  auto abiRole = ABIRoleInfo(VD);
+  if (!abiRole.providesAPI() && abiRole.getCounterpart())
+    return getDeclRepresentation(abiRole.getCounterpart(), isZeroSized);
+
   if (getActorIsolation(const_cast<ValueDecl *>(VD)).isActorIsolated())
     return {Unsupported, UnrepresentableIsolatedInActor};
   if (isa<MacroDecl>(VD))
diff --git a/lib/AST/TypeCheckRequests.cpp b/lib/AST/TypeCheckRequests.cpp
index 124329d967f81..d20c0b72d453f 100644
--- a/lib/AST/TypeCheckRequests.cpp
+++ b/lib/AST/TypeCheckRequests.cpp
@@ -356,7 +356,8 @@ void IsDynamicRequest::cacheResult(bool value) const {
   decl->setIsDynamic(value);
 
   // Add an attribute for printing
-  if (value && !decl->getAttrs().hasAttribute<DynamicAttr>())
+  if (value && !decl->getAttrs().hasAttribute<DynamicAttr>() &&
+        ABIRoleInfo(decl).providesAPI())
     decl->getAttrs().add(new (decl->getASTContext()) DynamicAttr(/*Implicit=*/true));
 }
 
diff --git a/lib/ClangImporter/ClangImporter.cpp b/lib/ClangImporter/ClangImporter.cpp
index e694108488f63..8eadf227e0591 100644
--- a/lib/ClangImporter/ClangImporter.cpp
+++ b/lib/ClangImporter/ClangImporter.cpp
@@ -6594,6 +6594,9 @@ findFunctionInterfaceAndImplementation(AbstractFunctionDecl *func) {
 
 ObjCInterfaceAndImplementation ObjCInterfaceAndImplementationRequest::
 evaluate(Evaluator &evaluator, Decl *decl) const {
+  ASSERT(ABIRoleInfo(decl).providesAPI()
+            && "@interface request for ABI-only decl?");
+
   // Types and extensions have direct links to their counterparts through the
   // `@_objcImplementation` attribute. Let's resolve that.
   // (Also directing nulls here, where they'll early-return.)
@@ -6633,6 +6636,21 @@ llvm::TinyPtrVector<Decl *> Decl::getAllImplementedObjCDecls() const {
     // This *is* the interface, if there is one.
     return {};
 
+  // ABI-only attributes don't have an `@implementation`, so query the API
+  // counterpart and map the results back to ABI decls.
+  auto abiRole = ABIRoleInfo(this);
+  if (!abiRole.providesAPI() && abiRole.getCounterpart()) {
+    auto interfaceDecls =
+        abiRole.getCounterpart()->getAllImplementedObjCDecls();
+
+    // Map the APIs back to their ABI counterparts (often a no-op)
+    for (auto &interfaceDecl : interfaceDecls) {
+      interfaceDecl = ABIRoleInfo(interfaceDecl).getCounterpart();
+    }
+
+    return interfaceDecls;
+  }
+
   ObjCInterfaceAndImplementationRequest req{const_cast<Decl *>(this)};
   auto result = evaluateOrDefault(getASTContext().evaluator, req, {});
   return result.interfaceDecls;
@@ -6650,6 +6668,14 @@ Decl *Decl::getObjCImplementationDecl() const {
     // This *is* the implementation, if it has one.
     return nullptr;
 
+  // ABI-only attributes don't have an `@implementation`, so query the API
+  // counterpart and map the results back to ABI decls.
+  auto abiRole = ABIRoleInfo(this);
+  if (!abiRole.providesAPI() && abiRole.getCounterpart()) {
+    auto implDecl = abiRole.getCounterpart()->getObjCImplementationDecl();
+    return ABIRoleInfo(implDecl).getCounterpart();
+  }
+
   ObjCInterfaceAndImplementationRequest req{const_cast<Decl *>(this)};
   auto result = evaluateOrDefault(getASTContext().evaluator, req, {});
   return result.implementationDecl;
diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index 9f3ebf072677f..a334b66bd25df 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -1422,6 +1422,8 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
 
   Opts.EnableSkipExplicitInterfaceModuleBuildRemarks = Args.hasArg(OPT_remark_skip_explicit_interface_build);
 
+  Opts.EnableABIInferenceRemarks = Args.hasArg(OPT_remark_abi_inference);
+
   if (Args.hasArg(OPT_experimental_skip_non_exportable_decls)) {
     // Only allow -experimental-skip-non-exportable-decls if either library
     // evolution is enabled (in which case the module's ABI is independent of
diff --git a/lib/Parse/ParseDecl.cpp b/lib/Parse/ParseDecl.cpp
index 7b770e0717c5a..2a85b94c3779c 100644
--- a/lib/Parse/ParseDecl.cpp
+++ b/lib/Parse/ParseDecl.cpp
@@ -2189,7 +2189,7 @@ Parser::parseAllowFeatureSuppressionAttribute(bool inverted, SourceLoc atLoc,
 
   auto range = SourceRange(loc, parensRange.End);
   return makeParserResult(AllowFeatureSuppressionAttr::create(
-      Context, loc, range, /*implicit*/ false, /*inverted*/ inverted,
+      Context, atLoc, range, /*implicit*/ false, /*inverted*/ inverted,
       features));
 }
 
@@ -9776,10 +9776,23 @@ Parser::parseDeclSubscript(SourceLoc StaticLoc,
 
   Decls.push_back(Subscript);
 
+  bool Invalid = false;
+  // Reject 'subscript' functions outside of type decls
+  if (!(Flags & PD_HasContainerType)) {
+    diagnose(SubscriptLoc, diag::subscript_decl_wrong_scope);
+    Invalid = true;
+  }
+
   // '{'
   // Parse getter and setter.
   ParsedAccessors accessors;
   if (Tok.isNot(tok::l_brace)) {
+    // Subscript stubs should never have accessors, and this one doesn't, so
+    // we're done.
+    if (Flags.contains(PD_StubOnly)) {
+      return makeParserResult(Status, Subscript);
+    }
+
     // Subscript declarations must always have at least a getter, so they need
     // to be followed by a {.
     if (!Status.isErrorOrHasCompletion()) {
@@ -9796,13 +9809,6 @@ Parser::parseDeclSubscript(SourceLoc StaticLoc,
                           Subscript);
   }
 
-  bool Invalid = false;
-  // Reject 'subscript' functions outside of type decls
-  if (!(Flags & PD_HasContainerType)) {
-    diagnose(SubscriptLoc, diag::subscript_decl_wrong_scope);
-    Invalid = true;
-  }
-
   accessors.record(*this, Subscript, (Invalid || !Status.isSuccess() ||
                                       Status.hasCodeCompletion()));
 
diff --git a/lib/SIL/IR/SILDeclRef.cpp b/lib/SIL/IR/SILDeclRef.cpp
index 56fa11d92b8f7..c8c714fef2149 100644
--- a/lib/SIL/IR/SILDeclRef.cpp
+++ b/lib/SIL/IR/SILDeclRef.cpp
@@ -451,6 +451,7 @@ static LinkageLimit getLinkageLimit(SILDeclRef constant) {
   using Kind = SILDeclRef::Kind;
 
   auto *d = constant.getDecl();
+  ASSERT(ABIRoleInfo(d).providesAPI() && "getLinkageLimit() for ABI decl?");
 
   // Back deployment thunks and fallbacks are emitted into the client.
   if (constant.backDeploymentKind != SILDeclRef::BackDeploymentKind::None)
@@ -1267,6 +1268,9 @@ std::string SILDeclRef::mangle(ManglingKind MKind) const {
     if (auto *ACE = getAbstractClosureExpr())
       return mangler.mangleClosureEntity(ACE, SKind);
 
+    ASSERT(ABIRoleInfo(getDecl()).providesAPI()
+              && "SILDeclRef mangling ABI decl directly?");
+
     // As a special case, functions can have manually mangled names.
     // Use the SILGen name only for the original non-thunked, non-curried entry
     // point.
diff --git a/lib/SIL/IR/SILFunctionBuilder.cpp b/lib/SIL/IR/SILFunctionBuilder.cpp
index 74647128c2872..b15f75b4645c8 100644
--- a/lib/SIL/IR/SILFunctionBuilder.cpp
+++ b/lib/SIL/IR/SILFunctionBuilder.cpp
@@ -377,6 +377,9 @@ SILFunction *SILFunctionBuilder::getOrCreateFunction(
     if (auto *accessor = dyn_cast<AccessorDecl>(decl)) {
       auto *storage = accessor->getStorage();
       // Add attributes for e.g. computed properties.
+      ASSERT(ABIRoleInfo(storage).providesAPI()
+                && "addFunctionAttributes() on ABI-only accessor?");
+
       addFunctionAttributes(F, storage->getAttrs(), mod,
                             getOrCreateDeclaration);
                             
@@ -395,6 +398,8 @@ SILFunction *SILFunctionBuilder::getOrCreateFunction(
         F->setInlineStrategy(NoInline);
       }
     }
+    ASSERT(ABIRoleInfo(decl).providesAPI()
+              && "addFunctionAttributes() on ABI-only decl?");
     addFunctionAttributes(F, decl->getAttrs(), mod, getOrCreateDeclaration,
                           constant);
   }
diff --git a/lib/SIL/IR/SILSymbolVisitor.cpp b/lib/SIL/IR/SILSymbolVisitor.cpp
index f58c424180c53..b45b48d7202ad 100644
--- a/lib/SIL/IR/SILSymbolVisitor.cpp
+++ b/lib/SIL/IR/SILSymbolVisitor.cpp
@@ -491,6 +491,9 @@ class SILSymbolVisitorImpl : public ASTVisitor<SILSymbolVisitorImpl> {
 
     addFunction(SILDeclRef(AFD));
 
+    ASSERT(ABIRoleInfo(AFD).providesAPI()
+              && "SILSymbolVisitorImpl visiting ABI-only decl?");
+
     if (auto dynKind = getDynamicKind(AFD)) {
       // Add the global function pointer for a dynamically replaceable function.
       Visitor.addDynamicFunction(AFD, *dynKind);
diff --git a/lib/SILGen/SILGen.cpp b/lib/SILGen/SILGen.cpp
index 0bd0a03e244ef..1a85e274fa077 100644
--- a/lib/SILGen/SILGen.cpp
+++ b/lib/SILGen/SILGen.cpp
@@ -1439,6 +1439,9 @@ void SILGenModule::emitAbstractFuncDecl(AbstractFunctionDecl *AFD) {
   // Emit default arguments and property wrapper initializers.
   emitArgumentGenerators(AFD, AFD->getParameters());
 
+  ASSERT(ABIRoleInfo(AFD).providesAPI()
+            && "emitAbstractFuncDecl() on ABI-only decl?");
+
   // If the declaration is exported as a C function, emit its native-to-foreign
   // thunk too, if it wasn't already forced.
   if (AFD->getAttrs().hasAttribute<CDeclAttr>()) {
diff --git a/lib/Sema/AssociatedTypeInference.cpp b/lib/Sema/AssociatedTypeInference.cpp
index 31a5c3285c1f6..720343abcca7e 100644
--- a/lib/Sema/AssociatedTypeInference.cpp
+++ b/lib/Sema/AssociatedTypeInference.cpp
@@ -3724,15 +3724,12 @@ bool AssociatedTypeInference::diagnoseNoSolutions(
               failed.Result.getKind() != CheckTypeWitnessResult::Superclass) {
             Type resultType;
             SourceRange typeRange;
-            if (auto *var = dyn_cast<VarDecl>(failed.Witness)) {
-              resultType = var->getValueInterfaceType();
-              typeRange = var->getTypeSourceRangeForDiagnostics();
+            if (auto *storage = dyn_cast<AbstractStorageDecl>(failed.Witness)) {
+              resultType = storage->getValueInterfaceType();
+              typeRange = storage->getTypeSourceRangeForDiagnostics();
             } else if (auto *func = dyn_cast<FuncDecl>(failed.Witness)) {
               resultType = func->getResultInterfaceType();
               typeRange = func->getResultTypeSourceRange();
-            } else if (auto *subscript = dyn_cast<SubscriptDecl>(failed.Witness)) {
-              resultType = subscript->getElementInterfaceType();
-              typeRange = subscript->getElementTypeSourceRange();
             }
 
             // If the type witness was inferred from an existential
diff --git a/lib/Sema/CMakeLists.txt b/lib/Sema/CMakeLists.txt
index 57df045386821..103435fa20d90 100644
--- a/lib/Sema/CMakeLists.txt
+++ b/lib/Sema/CMakeLists.txt
@@ -50,6 +50,7 @@ add_swift_host_library(swiftSema STATIC
   TypeOfReference.cpp
   TypeCheckAccess.cpp
   TypeCheckAttr.cpp
+  TypeCheckAttrABI.cpp
   TypeCheckAvailability.cpp
   TypeCheckBitwise.cpp
   TypeCheckCaptures.cpp
diff --git a/lib/Sema/TypeCheckAttr.cpp b/lib/Sema/TypeCheckAttr.cpp
index e3fc30db24619..1a6151c727c08 100644
--- a/lib/Sema/TypeCheckAttr.cpp
+++ b/lib/Sema/TypeCheckAttr.cpp
@@ -195,68 +195,10 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
   IGNORED_ATTR(Unsafe)
 #undef IGNORED_ATTR
 
-private:
-  static unsigned getABIArity(AbstractFunctionDecl *afd) {
-    unsigned arity = afd->getParameters()->size();
-    arity += afd->getGenericSignature().getGenericParams().size();
-    if (afd->hasImplicitSelfDecl())
-      arity += 1;
-    return arity;
-  }
-
-  void checkABIAttrPBD(PatternBindingDecl *APBD, VarDecl *VD) {
-    auto PBD = VD->getParentPatternBinding();
-
-    // To make sure we only diagnose this stuff once, check that VD is the first
-    // anchoring variable in the PBD.
-    bool isFirstAnchor = false;
-    for (auto i : range(PBD->getNumPatternEntries())) {
-      auto anchorVD = PBD->getAnchoringVarDecl(i);
-      if (anchorVD) {
-        isFirstAnchor = (anchorVD == VD);
-        break;
-      }
-    }
-
-    if (!isFirstAnchor)
-      return;
-
-    // Check that the PBDs have the same number of patterns.
-    if (PBD->getNumPatternEntries() < APBD->getNumPatternEntries()) {
-      diagnose(APBD->getPattern(PBD->getNumPatternEntries())->getLoc(),
-               diag::attr_abi_mismatched_pbd_size, /*abiHasExtra=*/false);
-      return;
-    }
-    if (PBD->getNumPatternEntries() > APBD->getNumPatternEntries()) {
-      diagnose(PBD->getPattern(APBD->getNumPatternEntries())->getLoc(),
-               diag::attr_abi_mismatched_pbd_size, /*abiHasExtra=*/true);
-      return;
-    }
-
-    // Check that each pattern has the same number of variables.
-    for (auto i : range(PBD->getNumPatternEntries())) {
-      SmallVector<VarDecl *, 8> VDs;
-      SmallVector<VarDecl *, 8> AVDs;
-
-      PBD->getPattern(i)->collectVariables(VDs);
-      APBD->getPattern(i)->collectVariables(AVDs);
-
-      if (VDs.size() < AVDs.size()) {
-        for (auto AVD : drop_begin(AVDs, VDs.size())) {
-          AVD->diagnose(diag::attr_abi_mismatched_var,
-                        AVD, /*isABI=*/true);
-        }
-      }
-      else if (VDs.size() > AVDs.size()) {
-        for (auto VD : drop_begin(VDs, AVDs.size())) {
-          VD->diagnose(diag::attr_abi_mismatched_var,
-                       VD, /*isABI=*/false);
-        }
-      }
-    }
+  void visitABIAttr(ABIAttr *attr) {
+    TypeChecker::checkDeclABIAttribute(D, attr);
   }
 
-public:
   void visitExecutionAttr(ExecutionAttr *attr) {
     auto *const decl = cast<ValueDecl>(D);
 
@@ -300,81 +242,6 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
     }
   }
 
-  void visitABIAttr(ABIAttr *attr) {
-    Decl *AD = attr->abiDecl;
-    if (isa<VarDecl>(D) && isa<PatternBindingDecl>(AD)) {
-      auto VD = cast<VarDecl>(D);
-      auto APBD = cast<PatternBindingDecl>(AD);
-
-      // Diagnose dissimilar PBD structures.
-      checkABIAttrPBD(APBD, VD);
-
-      // Do the rest of this checking on the corresponding VarDecl, not the
-      // PBD that's actually in the attribute. Note that `AD` will become null
-      // if they're too dissimilar to match up.
-      AD = APBD->getVarAtSimilarStructuralPosition(VD);
-    }
-    // TODO: EnumElementDecl?
-
-    if (!AD)
-      return;
-
-    // Check the ABI decl and bail if there was a problem with it.
-    TypeChecker::typeCheckDecl(AD);
-    if (AD->isInvalid())
-      return;
-
-    // Do the declarations have the same kind, broadly speaking? Many kinds have
-    // special mangling behavior (e.g. inits vs normal funcs) that make it
-    // unrealistic to treat one kind as though it were another.
-    if (D->getKind() != AD->getKind()) {
-      // FIXME: DescriptiveDeclKind is overly specific; we really just want to
-      //        say that e.g. a `func` can't have the ABI of a `var`.
-      diagnoseAndRemoveAttr(attr, diag::attr_abi_mismatched_kind,
-                            D, AD->getDescriptiveKind());
-      return;
-    }
-
-    if (isa<AbstractFunctionDecl>(D)) {
-      auto AFD = cast<AbstractFunctionDecl>(D);
-      auto AAFD = cast<AbstractFunctionDecl>(AD);
-
-      // FIXME: How much should we diagnose in IRGen for more precise ABI info?
-
-      // Do the declarations have roughly the same number of parameters? We'll
-      // allow some fuzziness for what these parameters *are*, since there isn't
-      // always an ABI difference between e.g. a free function with N parameters
-      // and an instance method with N-1 parameters (plus an implicit `self`).
-      if (getABIArity(AFD) != getABIArity(AAFD)) {
-        diagnoseAndRemoveAttr(attr, diag::attr_abi_mismatched_arity,
-                              AFD);
-      }
-
-      // Do the declarations match in throwing behavior? We don't care about
-      // `throws` vs. `rethrows` here, just whether callers will account for an
-      // error return.
-      // FIXME: Typed throws?
-      if (AFD->hasThrows() != AAFD->hasThrows()) {
-        diagnoseAndRemoveAttr(attr, diag::attr_abi_mismatched_throws,
-                              AFD, /*abiCanThrow=*/AAFD->hasThrows());
-      }
-
-      // Do the declarations match in async-ness?
-      if (AFD->hasAsync() != AAFD->hasAsync()) {
-        diagnoseAndRemoveAttr(attr, diag::attr_abi_mismatched_async,
-                              AFD, /*abiHasAsync=*/AAFD->hasAsync());
-      }
-    }
-
-    // TODO: Diagnose if Protocol::isMarkerProtocol() - contradiction in terms
-    //       (and mangler can't handle invertible protocols with @abi)
-
-    // TODO: Validate more
-    // FIXME: The list of properties that have to match is practically endless
-    // and will grow as new features are added to the compiler. We might want to
-    // write an AttributeVisitor just to help us catch omissions over time.
-  }
-
   void visitAlignmentAttr(AlignmentAttr *attr) {
     // Alignment must be a power of two.
     auto value = attr->getValue();
@@ -1795,6 +1662,10 @@ static SourceRange getArgListRange(ASTContext &Ctx, DeclAttribute *attr) {
 
 void AttributeChecker::
 visitObjCImplementationAttr(ObjCImplementationAttr *attr) {
+  // If `D` is ABI-only, let ABIDeclChecker diagnose the bad attribute.
+  if (!ABIRoleInfo(D).providesAPI())
+    return;
+
   DeclAttribute * langAttr =
     D->getAttrs().getAttribute<ObjCAttr>(/*AllowInvalid=*/true);
   if (!langAttr)
@@ -5267,7 +5138,8 @@ void AttributeChecker::checkBackDeployedAttrs(
     }
 
     if (auto *AFD = dyn_cast<AbstractFunctionDecl>(D)) {
-      if (!AFD->hasBody()) {
+      // Ignore this for ABI-only decls; ABIDeclChecker will diagnose it better.
+      if (!AFD->hasBody() && ABIRoleInfo(AFD).providesAPI()) {
         diagnoseAndRemoveAttr(Attr, diag::back_deployed_requires_body, Attr,
                               VD);
         continue;
@@ -6800,6 +6672,12 @@ static bool typeCheckDerivativeAttr(DerivativeAttr *attr) {
   // Note: Implementation must be idempotent because it may be called multiple
   // times for the same attribute.
   Decl *D = attr->getOriginalDeclaration();
+
+  // ABI-only decls can't have @derivative; bail out and let ABIDeclChecker
+  // diagnose this.
+  if (!ABIRoleInfo(D).providesAPI())
+    return false;
+
   auto &Ctx = D->getASTContext();
   auto &diags = Ctx.Diags;
   // `@derivative` attribute requires experimental differentiable programming
diff --git a/lib/Sema/TypeCheckAttrABI.cpp b/lib/Sema/TypeCheckAttrABI.cpp
new file mode 100644
index 0000000000000..d81c018ed7faa
--- /dev/null
+++ b/lib/Sema/TypeCheckAttrABI.cpp
@@ -0,0 +1,1205 @@
+//===--- TypeCheckAttrABI.cpp - Type Checking for @abi Attribute ----------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2024 - 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// This file implements diagnostics for the @abi attribute.
+///
+//===----------------------------------------------------------------------===//
+
+#include "TypeChecker.h"
+#include "swift/AST/Decl.h"
+#include "swift/AST/Effects.h"
+#include "swift/AST/DiagnosticsSema.h"
+#include "swift/AST/ParameterList.h"
+#include "swift/AST/ASTPrinter.h"
+#include "swift/AST/Types.h"
+#include "swift/Basic/Assertions.h"
+#include "swift/Parse/Lexer.h"
+#include <optional>
+
+using namespace swift;
+
+namespace {
+
+/// Like ASTVisitor, but the visit methods are passed a pair of nodes to compare.
+/// Either node might be \c nil , indicating that a matching node was not found.
+template <typename ImplClass>
+class ASTComparisonVisitor {
+public:
+  bool visit(Decl *D1, Decl *D2) {
+    DeclKind kind = D1 ? D1->getKind() : D2->getKind();
+    switch (kind) {
+#define DECL(CLASS, PARENT) \
+    case DeclKind::CLASS: \
+      return static_cast<ImplClass*>(this) \
+        ->visit##CLASS##Decl(static_cast<CLASS##Decl*>(D1), \
+                             static_cast<CLASS##Decl*>(D2));
+#include "swift/AST/DeclNodes.def"
+    }
+    llvm_unreachable("Not reachable, all cases handled");
+  }
+
+#define DECL(CLASS, PARENT) \
+  bool visitParentOf##CLASS##Decl(CLASS##Decl *D1, CLASS##Decl *D2) {\
+    return static_cast<ImplClass*>(this)->visit##PARENT(D1, D2); \
+  }
+#define ABSTRACT_DECL(CLASS, PARENT) DECL(CLASS, PARENT)
+#include "swift/AST/DeclNodes.def"
+};
+
+/// Describes the effects that have been applied to a declaration, packaging up
+/// various bits of info used for \c \@abi diagnostics.
+struct DeclEffects {
+  PossibleEffects effects;
+  PossibleEffects polymorphicEffects;
+
+  SourceLoc asyncLoc;
+  SourceLoc throwsLoc;
+
+  std::optional<Type> effectiveThrownType;
+  TypeRepr *thrownTypeRepr;
+
+  DeclEffects(AbstractFunctionDecl *afd)
+    : effects(), polymorphicEffects(),
+      asyncLoc(afd->getAsyncLoc()), throwsLoc(afd->getThrowsLoc()),
+      effectiveThrownType(afd->getEffectiveThrownErrorType()),
+      thrownTypeRepr(afd->getThrownTypeRepr())
+  {
+    if (afd->hasEffect(EffectKind::Async))
+      effects |= EffectKind::Async;
+    if (afd->hasPolymorphicEffect(EffectKind::Async))
+      polymorphicEffects |= EffectKind::Async;
+
+    if (afd->hasEffect(EffectKind::Throws))
+      effects |= EffectKind::Throws;
+    if (afd->hasPolymorphicEffect(EffectKind::Throws))
+      polymorphicEffects |= EffectKind::Throws;
+  }
+
+  bool anyContains(EffectKind effect) const {
+    return effects.contains(effect) || polymorphicEffects.contains(effect);
+  }
+};
+
+/// Describes the relationship between a given type and the declaration it
+/// belongs to--e.g. is this its result type? a parameter type? etc. Used with
+/// a couple of \c \@abi diagnostics.
+class TypeOrigin {
+public:
+  // Cases must be kept in sync with DiagnosticsSema TYPE_ORIGIN
+  enum class Kind : uint8_t {
+    Unspecified = 0,
+    Parameter = 1,
+    SelfParameter = 2,
+    Result = 3,
+    ThrowsEffect = 4,
+  };
+
+private:
+  llvm::PointerIntPair<Decl *, 3, Kind> declAndKind;
+
+  TypeOrigin(Decl *decl, Kind kind)
+    : declAndKind(decl, kind) {}
+
+public:
+  static TypeOrigin forUnspecified() {
+    return TypeOrigin(nullptr, Kind::Unspecified);
+  }
+
+  static TypeOrigin forParameter(ParamDecl *paramDecl) {
+    return TypeOrigin(paramDecl,
+                      paramDecl->isSelfParameter() ? Kind::SelfParameter
+                                                   : Kind::Parameter);
+  }
+
+  static TypeOrigin forResult() {
+    return TypeOrigin(nullptr, Kind::Result);
+  }
+
+  static TypeOrigin forThrowsEffect() {
+    return TypeOrigin(nullptr, Kind::ThrowsEffect);
+  }
+
+  Kind getKind() const {
+    return declAndKind.getInt();
+  }
+
+  Decl *getDecl() const {
+    return declAndKind.getPointer();
+  }
+};
+
+/// Emit a fix-it replacing \p charRange with \p newText , inserting or
+/// removing whitespace after \c charRange in a way suitable for editing a
+/// sequence of whitespce-separated keywords.
+void fixItReplaceKeywords(InFlightDiagnostic &diag,
+                          CharSourceRange charRange,
+                          StringRef newText) {
+  auto &SM = diag.getSourceManager();
+  auto charRangeIsFollowedByWhitespace = [&]() -> bool {
+    auto str = SM.extractText({ charRange.getEnd(), 1 });
+    return str.empty() ? false : isspace(str.front());
+  };
+
+  SmallString<32> scratch;
+  scratch += newText;
+
+  if (newText.empty()) {
+    // Eat trailing whitespace.
+    while (charRangeIsFollowedByWhitespace()) {
+      charRange = { charRange.getStart(), charRange.getByteLength() + 1 };
+    }
+  } else {
+    if (!charRangeIsFollowedByWhitespace()) {
+      scratch.push_back(' ');
+    }
+  }
+
+  diag.fixItReplaceChars(charRange.getStart(), charRange.getEnd(), scratch);
+}
+
+/// Emit a fix-it replacing \p range with \p newText , inserting or
+/// removing whitespace after \c range in a way suitable for editing a
+/// sequence of whitespce-separated keywords.
+void fixItReplaceKeywords(InFlightDiagnostic &diag,
+                          SourceRange range,
+                          StringRef newText) {
+  auto &SM = diag.getSourceManager();
+  auto charRange = Lexer::getCharSourceRangeFromSourceRange(SM, range);
+  return fixItReplaceKeywords(diag, charRange, newText);
+}
+
+/// Returns a string representation of \p attr suitable to either replace an
+/// existing attribute or be inserted as a new attribute, depending on the
+/// value of \p toInsert .
+StringRef printAttr(DeclAttribute *attr,
+                    Decl *decl,
+                    SmallVectorImpl<char> &scratch,
+                    bool toInsert = false) {
+  auto &ctx = decl->getASTContext();
+  auto opts = PrintOptions::printForDiagnostics(AccessLevel::Private,
+                              ctx.TypeCheckerOpts.PrintFullConvention);
+  opts.PrintLongAttrsOnSeparateLines = false;
+
+  llvm::raw_svector_ostream os{scratch};
+  StreamPrinter printer{os};
+  attr->print(printer, opts, decl);
+
+  auto str = StringRef(scratch.begin(), scratch.size());
+  if (!toInsert)
+    str = str.trim(' ');
+  return str;
+}
+
+/// Emit \c diag::attr_abi_matching_attr_here in the best available location.
+void noteAttrHere(DeclAttribute *attr, Decl *decl, bool isMatch = false) {
+  auto &ctx = decl->getASTContext();
+  SourceLoc loc = attr->getLocation();
+  if (loc.isValid())
+    ctx.Diags.diagnose(loc, diag::attr_abi_matching_attr_here,
+                       isMatch, attr->isDeclModifier(), attr->isImplicit());
+  else
+    ctx.Diags.diagnose(decl, diag::attr_abi_matching_attr_here,
+                       isMatch, attr->isDeclModifier(), attr->isImplicit());
+}
+
+/// Get the best available \c SourceLoc representing the type in \p storage .
+SourceLoc getTypeLoc(AbstractStorageDecl *storage, Decl *owner = nullptr) {
+  auto loc = storage->getTypeSourceRangeForDiagnostics().Start;
+  if (loc.isInvalid())
+    loc = storage->getLoc();
+  if (loc.isInvalid() && owner)
+    loc = owner->getLoc();
+  return loc;
+}
+
+/// Get a decl's generic signature, if it has one.
+GenericSignature getGenericSignature(Decl *decl) {
+  if (auto genericCtx = decl->getAsGenericContext())
+    return genericCtx->getGenericSignature();
+  return GenericSignature();
+}
+
+class ABIDeclChecker : public ASTComparisonVisitor<ABIDeclChecker> {
+  ASTContext &ctx;
+  Decl *diagnoseOnDecl;
+  ABIAttr *abiAttr;
+
+  /// Used to de-duplicate short-form \c \@available attrs. See \c checkAttr() .
+  SmallSetVector<SourceLoc, 4> diagnosedAvailableAttrSourceLocs;
+
+  /// This emits a diagnostic with a fixit to remove the attribute.
+  template<typename ...ArgTypes>
+  InFlightDiagnostic diagnoseAndRemoveAttr(DeclAttribute *attr,
+                                           ArgTypes &&...Args) {
+    return swift::diagnoseAndRemoveAttr(diagnoseOnDecl, attr,
+                                        std::forward<ArgTypes>(Args)...);
+  }
+
+public:
+  ABIDeclChecker(ASTContext &ctx, Decl *diagnoseOnDecl, ABIAttr *abiAttr)
+    : ctx(ctx), diagnoseOnDecl(diagnoseOnDecl), abiAttr(abiAttr) {}
+
+  // MARK: @abi checking - decls
+
+  void check(Decl *api, Decl *abi) {
+    // Do the declarations have the same kind, broadly speaking? Many kinds have
+    // special mangling behavior (e.g. inits vs normal funcs) that make it
+    // unrealistic to treat one kind as though it were another.
+    // (And if they don't, we can't really compare them properly.
+    if (api->getKind() != abi->getKind()) {
+      // FIXME: DescriptiveDeclKind is overly specific; we really just want to
+      //        say that e.g. a `func` can't have the ABI of a `var`.
+      diagnoseAndRemoveAttr(abiAttr, diag::attr_abi_mismatched_kind,
+                            api, abi->getDescriptiveKind());
+      return;
+    }
+
+    visit(api, abi);
+  }
+
+  bool checkParameterFlags(ParameterTypeFlags api, ParameterTypeFlags abi,
+                           ParameterTypeFlags apiOrig,
+                           ParameterTypeFlags abiOrig,
+                           Type apiType, Type abiType,
+                           SourceLoc apiTypeLoc, SourceLoc abiTypeLoc,
+                           TypeOrigin origin) {
+    // Some keywords are spelled differently for a `self` parameter.
+    bool isSelfParam = origin.getKind() == TypeOrigin::Kind::SelfParameter;
+
+    bool didDiagnose = false;
+
+    auto noteShouldMatch = [&](bool isModifier) {
+      if (isSelfParam)
+        ctx.Diags.diagnose(apiTypeLoc, diag::attr_abi_matching_attr_here,
+                           /*matches=*/false, isModifier, /*isImplicit=*/false);
+      else
+        ctx.Diags.diagnose(apiTypeLoc, diag::attr_abi_should_match_type_here);
+    };
+
+    // These assertions represent values that should have been normalized.
+    ASSERT(!api.isVariadic() && !abi.isVariadic());
+    ASSERT(!api.isAutoClosure() && !abi.isAutoClosure());
+    ASSERT(!api.isNonEphemeral() && !abi.isNonEphemeral());
+    ASSERT(!api.isIsolated() && !abi.isIsolated());
+    ASSERT(!api.isSending() && !abi.isSending());
+    ASSERT(!api.isCompileTimeLiteral() && !abi.isCompileTimeLiteral());
+
+    if (api.getOwnershipSpecifier() != abi.getOwnershipSpecifier()) {
+      auto getSpelling = [=](ParamSpecifier spec) -> StringRef {
+        // Customize a couple of names to match what the developer would
+        // actually write.
+        if (spec == ParamSpecifier::Default)
+          return "";
+        if (spec == ParamSpecifier::InOut && isSelfParam)
+          return "mutating";
+        if (spec == ParamSpecifier::LegacyOwned && isSelfParam)
+          return "__consuming";
+        if (spec == ParamSpecifier::ImplicitlyCopyableConsuming)
+          return "sending";
+        return getNameForParamSpecifier(spec);
+      };
+
+      ctx.Diags.diagnose(abiTypeLoc, diag::attr_abi_mismatched_param_modifier,
+                         getSpelling(abiOrig.getOwnershipSpecifier()),
+                         getSpelling(apiOrig.getOwnershipSpecifier()),
+                         /*isModifier=*/true, unsigned(origin.getKind()),
+                         origin.getDecl());
+      noteShouldMatch(/*isModifier=*/true);
+      didDiagnose = true;
+    }
+
+    if (api.isNoDerivative() != abi.isNoDerivative()) {
+      ctx.Diags.diagnose(abiTypeLoc, diag::attr_abi_mismatched_param_modifier,
+                         abiOrig.isNoDerivative() ? "noDerivative" : "",
+                         apiOrig.isNoDerivative() ? "noDerivative" : "",
+                         /*isModifier=*/false, unsigned(origin.getKind()),
+                         origin.getDecl());
+      noteShouldMatch(/*isModifier=*/false);
+      didDiagnose = true;
+    }
+
+    if (api.isAddressable() != abi.isAddressable()) {
+      StringRef spelling = isSelfParam ? "_addressableSelf" : "_addressable";
+      ctx.Diags.diagnose(abiTypeLoc, diag::attr_abi_mismatched_param_modifier,
+                         abiOrig.isAddressable() ? spelling : "",
+                         apiOrig.isAddressable() ? spelling : "",
+                         /*isModifier=*/false, unsigned(origin.getKind()),
+                         origin.getDecl());
+      noteShouldMatch(/*isModifier=*/false);
+      didDiagnose = true;
+    }
+
+    if (!didDiagnose && api != abi) {
+      // Flag difference not otherwise diagnosed. This is a fallback diagnostic.
+      ctx.Diags.diagnose(abiTypeLoc, diag::attr_abi_mismatched_type,
+                         unsigned(origin.getKind()), origin.getDecl(),
+                         abiType, apiType);
+      ctx.Diags.diagnose(apiTypeLoc, diag::attr_abi_should_match_type_here);
+      didDiagnose = true;
+    }
+
+    return didDiagnose;
+  }
+
+  bool checkParameter(ParamDecl *api, ParamDecl *abi,
+                      ValueDecl *apiDecl, ValueDecl *abiDecl) {
+    ASSERT(api && abi);
+
+    bool didDiagnose = false;
+    if (auto defaultExpr = abi->getStructuralDefaultExpr()) {
+      // Forbidden.
+      ctx.Diags.diagnose(defaultExpr->getLoc(),
+                         diag::attr_abi_no_default_arguments, abi);
+      // TODO: Fix removing default arg (requires SourceLoc for equal sign)
+
+      // Don't return immediately because we can independently check the type.
+      didDiagnose = true;
+    }
+
+    auto apiOrig = api->toFunctionParam();
+    auto abiOrig = abi->toFunctionParam();
+    // FIXME: Do `self` params have the same default param specifier behavior?
+    auto apiNorm = normalizeParam(apiOrig, apiDecl);
+    auto abiNorm = normalizeParam(abiOrig, abiDecl);
+
+    // FIXME: Refine to point to specific modifiers where possible.
+    SourceLoc apiTypeLoc = getTypeLoc(api, apiDecl);
+    SourceLoc abiTypeLoc = getTypeLoc(abi, abiDecl);
+
+    didDiagnose |= checkType(apiNorm.getPlainType(), abiNorm.getPlainType(),
+                             apiTypeLoc, abiTypeLoc,
+                             getGenericSignature(apiDecl),
+                             getGenericSignature(abiDecl),
+                             TypeOrigin::forParameter(abi));
+
+    didDiagnose |= checkParameterFlags(apiNorm.getParameterFlags(),
+                                       abiNorm.getParameterFlags(),
+                                       apiOrig.getParameterFlags(),
+                                       abiOrig.getParameterFlags(),
+                                       apiNorm.getPlainType(),
+                                       abiNorm.getPlainType(),
+                                       apiTypeLoc, abiTypeLoc,
+                                       TypeOrigin::forParameter(abi));
+
+    didDiagnose |= checkAttrs(api->getAttrs(), abi->getAttrs(), api, abi);
+
+    return didDiagnose;
+  }
+
+  bool checkParameterList(ParameterList *api, ParameterList *abi,
+                          ValueDecl *apiDecl, ValueDecl *abiDecl) {
+    // Do the declarations have the same number of parameters?
+    if (api->size() != abi->size()) {
+      diagnoseAndRemoveAttr(abiAttr, diag::attr_abi_mismatched_arity, apiDecl,
+                            /*genericParams=*/false);
+      return true;
+    }
+
+    bool didDiagnose = false;
+
+    for (auto pair : llvm::zip(*api, *abi)) {
+      didDiagnose |= checkParameter(std::get<0>(pair), std::get<1>(pair),
+                                    apiDecl, abiDecl);
+    }
+
+    return didDiagnose;
+  }
+
+  bool checkImplicitSelfParam(ParamDecl *api, ParamDecl *abi,
+                              ValueDecl *apiDecl, ValueDecl *abiDecl) {
+    if (!api && !abi)
+      // Nothing to check
+      return false;
+
+    if ((api && !abi) || (!api && abi)) {
+      diagnoseAndRemoveAttr(abiAttr, diag::attr_abi_mismatched_arity,
+                            apiDecl, /*genericParams=*/false);
+      return true;
+    }
+
+    return checkParameter(api, abi, apiDecl, abiDecl);
+  }
+
+  bool checkGenericSignature(GenericSignature api, GenericSignature abi,
+                             Decl *apiDecl, Decl *abiDecl) {
+    if (api.isNull() && abi.isNull())
+      return false;
+
+    if (api.isNull()) {
+      abiDecl->diagnose(diag::attr_abi_extra_generic_signature, apiDecl);
+      return true;
+    }
+
+    if (abi.isNull()) {
+      abiDecl->diagnose(diag::attr_abi_missing_generic_signature,
+                        api.getAsString());
+      return true;
+    }
+
+    auto apiNorm = normalizeGenericSignature(api);
+    auto abiNorm = normalizeGenericSignature(abi);
+
+    if (!apiNorm->isEqual(abiNorm)) {
+      abiDecl->diagnose(diag::attr_abi_mismatched_generic_signature,
+                        abi.getAsString(), api.getAsString());
+      apiDecl->diagnose(diag::attr_abi_should_match_type_here);
+      return true;
+    }
+
+    return false;
+  }
+
+  bool checkEffects(DeclEffects api, DeclEffects abi, Decl *apiDecl,
+                    Decl *abiDecl) {
+    bool didDiagnose = false;
+
+    // Do the declarations match in throwing behavior? We don't care about
+    // `throws` vs. `rethrows` here, just whether callers will account for an
+    // error return.
+    bool apiThrows = api.anyContains(EffectKind::Throws);
+    bool abiThrows = abi.anyContains(EffectKind::Throws);
+
+    if (apiThrows != abiThrows) {
+      diagnoseAndRemoveAttr(abiAttr, diag::attr_abi_mismatched_throws,
+                            apiDecl, /*abiCanThrow=*/abiThrows);
+      didDiagnose = true;
+    } else if (apiThrows && abiThrows) {
+      // If both throw, make sure the throw types are compatible.
+      auto apiThrowType = api.effectiveThrownType.value_or(ctx.getNeverType());
+      auto abiThrowType = abi.effectiveThrownType.value_or(ctx.getNeverType());
+
+      didDiagnose |= checkType(apiThrowType, abiThrowType,
+                               api.throwsLoc, abi.throwsLoc,
+                               getGenericSignature(apiDecl),
+                               getGenericSignature(abiDecl),
+                               TypeOrigin::forThrowsEffect());
+    }
+
+    // Do the declarations match in async-ness?
+    if (api.anyContains(EffectKind::Async) != abi.anyContains(EffectKind::Async)) {
+      diagnoseAndRemoveAttr(abiAttr, diag::attr_abi_mismatched_async,
+                            apiDecl, /*abiHasAsync=*/abi.anyContains(EffectKind::Async));
+      didDiagnose = true;
+    }
+
+    return didDiagnose;
+  }
+
+  bool checkFailable(ConstructorDecl *api, ConstructorDecl *abi) {
+    if (api->isFailable() == abi->isFailable()) {
+      return false;
+    }
+
+    auto diag = ctx.Diags.diagnose(abiAttr->getLocation(),
+                                   diag::attr_abi_failable_mismatch, api,
+                                   api->isFailable(), abi->isFailable());
+
+    if (api->isFailable())
+      diag.fixItInsertAfter(abi->getLoc(),
+                            api->isImplicitlyUnwrappedOptional() ? "!" : "?");
+    else
+      diag.fixItRemove(abi->getFailabilityLoc());
+
+    return true;
+  }
+
+  bool checkStaticAndFinal(ValueDecl *api, ValueDecl *abi) {
+    // The `static`, `class`, and `final` keywords all need to be evaluated
+    // together because of their intertwined semantics. This pile of code
+    // diagnoses errors in either or both.
+
+    /// Returns the type and location of the declaration's `static` or `class` keyword, if any.
+    auto getStaticSpelling = [](Decl *decl) -> Located<StaticSpellingKind> {
+      if (auto var = dyn_cast<VarDecl>(decl))
+        decl = var->getParentPatternBinding();
+
+      if (auto pbd = dyn_cast<PatternBindingDecl>(decl))
+        return { pbd->getStaticSpelling(), pbd->getStaticLoc() };
+
+      if (auto subscript = dyn_cast<SubscriptDecl>(decl))
+        return { subscript->getStaticSpelling(), subscript->getStaticLoc() };
+
+      if (auto func = dyn_cast<FuncDecl>(decl))
+        return { func->getStaticSpelling(), func->getStaticLoc() };
+
+      return { StaticSpellingKind::None, SourceLoc() };
+    };
+
+    /// Represents the combination of `class`, `static`, and `final` keywords
+    /// for a given declaration.
+    enum class StaticnessAndFinality : uint8_t {
+      InstanceAndOverridable,
+      ClassAndOverridable,
+      InstanceAndFinal,
+      ClassAndFinal,
+      Static,
+    };
+
+    /// Returns a `StaticnessAndFinality` corresponding to the given values.
+    auto getStaticnessAndFinality = [](StaticSpellingKind staticSpelling,
+                                       bool isFinal) {
+      switch (staticSpelling) {
+      case StaticSpellingKind::None:
+        return isFinal ? StaticnessAndFinality::InstanceAndFinal
+                       : StaticnessAndFinality::InstanceAndOverridable;
+
+      case StaticSpellingKind::KeywordStatic:
+        return StaticnessAndFinality::Static;
+
+      case StaticSpellingKind::KeywordClass:
+        return isFinal ? StaticnessAndFinality::ClassAndFinal
+                       : StaticnessAndFinality::ClassAndOverridable;
+      }
+
+      llvm_unreachable("unknown StaticSpellingKind");
+    };
+
+    auto apiSAF = getStaticnessAndFinality(getStaticSpelling(api).Item,
+                                           api->isFinal());
+
+    auto abiStaticSpelling = getStaticSpelling(abi);
+    auto abiSAF = getStaticnessAndFinality(abiStaticSpelling.Item,
+                                           abi->isFinal());
+
+    /// Collapses down the difference between `Static` and `ClassAndFinal`.
+    auto getSemantics = [](StaticnessAndFinality syntax) {
+      if (syntax == StaticnessAndFinality::ClassAndFinal)
+        return StaticnessAndFinality::Static;
+      return syntax;
+    };
+
+    if (getSemantics(apiSAF) != getSemantics(abiSAF)) {
+      auto diag = abi->diagnose(diag::attr_abi_static_final_mismatch,
+                                uint8_t(abiSAF), abi, uint8_t(apiSAF), api,
+                                api->getDeclContext()->getSelfClassDecl());
+
+      SourceLoc insertLoc = abi->getAttributeInsertionLoc(/*forModifier=*/true);
+      SourceLoc replaceLoc = abiStaticSpelling.Loc;
+      SourceLoc deleteLoc;
+
+      // If there's a (non-implicit) `final`, we may want to fix it.
+      auto finalAttr = abi->getAttrs().getAttribute<FinalAttr>();
+      if (finalAttr && !finalAttr->isImplicit())
+        deleteLoc = finalAttr->getLocation();
+
+      // If only one is valid, that should be `replaceLoc`; if both are valid,
+      // `replaceLoc` should come before `deleteLoc`.
+      if (deleteLoc.isValid() && (replaceLoc.isInvalid() ||
+                                 ctx.SourceMgr.isBefore(deleteLoc, replaceLoc)))
+        std::swap(deleteLoc, replaceLoc);
+
+      // Delete the keyword at `deleteLoc`, if there is one.
+      if (deleteLoc.isValid())
+        fixItReplaceKeywords(diag, deleteLoc, "");
+
+      StringRef spellings[] = { "", "class", "final", "final class", "static" };
+      auto newKeywords = spellings[uint8_t(apiSAF)];
+
+      // Either replace the first keyword, or insert new keywords.
+      if (replaceLoc.isValid())
+        fixItReplaceKeywords(diag, replaceLoc, newKeywords);
+      else
+        fixItReplaceKeywords(diag, CharSourceRange(insertLoc, 0), newKeywords);
+
+      return true;
+    }
+
+    return false;
+  }
+
+  /// This declaration should not be in an `@abi` attribute.
+#define UNSUPPORTED_DECL(NAME) \
+  bool visit##NAME##Decl(NAME##Decl *api, NAME##Decl *abi) { \
+    return visitParentOf##NAME##Decl(api, abi); \
+  }
+
+  /// This declaration has no additional validation logic.
+#define PASSTHROUGH_DECL(NAME) \
+  bool visit##NAME##Decl(NAME##Decl *api, NAME##Decl *abi) { \
+    return visitParentOf##NAME##Decl(api, abi); \
+  }
+
+  bool visitDecl(Decl *api, Decl *abi) {
+    bool didDiagnose = checkAttrs(api->getAttrs(), abi->getAttrs(), api, abi);
+
+    if (api->getAsGenericContext()) {
+      didDiagnose |= checkGenericSignature(getGenericSignature(api),
+                                           getGenericSignature(abi),
+                                           api, abi);
+    }
+
+    return didDiagnose;
+  }
+
+  bool visitValueDecl(ValueDecl *api, ValueDecl *abi) {
+    if (visitParentOfValueDecl(api, abi))
+      return true;
+
+    return checkStaticAndFinal(api, abi);
+  }
+
+  PASSTHROUGH_DECL(Type)
+  PASSTHROUGH_DECL(GenericType)
+  PASSTHROUGH_DECL(NominalType)
+  PASSTHROUGH_DECL(Operator)
+
+  UNSUPPORTED_DECL(Enum)
+  UNSUPPORTED_DECL(Struct)
+  UNSUPPORTED_DECL(Class)
+
+  // TODO: When supported, diagnose if Protocol::isMarkerProtocol()
+  //       (mangler can't handle invertible protocols with @abi)
+  UNSUPPORTED_DECL(Protocol)
+
+  UNSUPPORTED_DECL(BuiltinTuple)
+  UNSUPPORTED_DECL(OpaqueType)
+  UNSUPPORTED_DECL(TypeAlias)
+  UNSUPPORTED_DECL(GenericTypeParam)
+  UNSUPPORTED_DECL(AssociatedType)
+  UNSUPPORTED_DECL(Module)
+  UNSUPPORTED_DECL(Param)
+  UNSUPPORTED_DECL(Destructor)
+  UNSUPPORTED_DECL(Macro)
+  UNSUPPORTED_DECL(EnumElement)
+  UNSUPPORTED_DECL(Extension)
+  UNSUPPORTED_DECL(TopLevelCode)
+  UNSUPPORTED_DECL(Import)
+  UNSUPPORTED_DECL(PrecedenceGroup)
+  UNSUPPORTED_DECL(Missing)
+  UNSUPPORTED_DECL(MissingMember)
+  UNSUPPORTED_DECL(PatternBinding)
+  UNSUPPORTED_DECL(EnumCase)
+  UNSUPPORTED_DECL(Accessor)
+  UNSUPPORTED_DECL(InfixOperator)
+  UNSUPPORTED_DECL(PrefixOperator)
+  UNSUPPORTED_DECL(PostfixOperator)
+  UNSUPPORTED_DECL(MacroExpansion)
+
+  bool visitAbstractFunctionDecl(AbstractFunctionDecl *api,
+                                 AbstractFunctionDecl *abi) {
+    if (visitParentOfAbstractFunctionDecl(api, abi))
+      return true;
+
+    // FIXME: How much should we diagnose in IRGen for more precise ABI info?
+
+    if (checkImplicitSelfParam(api->getImplicitSelfDecl(),
+                               abi->getImplicitSelfDecl(),
+                               api, abi))
+      return true;
+
+    if (checkParameterList(api->getParameters(), abi->getParameters(),
+                           api, abi))
+      return true;
+
+    return checkEffects(DeclEffects(api), DeclEffects(abi), api, abi);
+    // NOTE: Does not check result type--that's the subclass's responsibility!
+  }
+
+  bool visitFuncDecl(FuncDecl *api, FuncDecl *abi) {
+    if (visitParentOfFuncDecl(api, abi))
+      return true;
+
+    // Intentionally ignoring `hasSendingResult()` because it doesn't affect
+    // calling convention.
+
+    return checkType(api->getResultInterfaceType(),
+                     abi->getResultInterfaceType(),
+                     api->getResultTypeSourceRange().Start,
+                     abi->getResultTypeSourceRange().Start,
+                     getGenericSignature(api), getGenericSignature(abi),
+                     TypeOrigin::forResult());
+  }
+
+  bool visitConstructorDecl(ConstructorDecl *api, ConstructorDecl *abi) {
+    if (visitParentOfConstructorDecl(api, abi))
+      return true;
+
+    return checkFailable(api, abi);
+  }
+
+  bool visitAbstractStorageDecl(AbstractStorageDecl *api,
+                                AbstractStorageDecl *abi) {
+    if (visitParentOfAbstractStorageDecl(api, abi))
+      return true;
+
+    if (checkType(api->getValueInterfaceType(), abi->getValueInterfaceType(),
+                  getTypeLoc(api), getTypeLoc(abi),
+                  getGenericSignature(api), getGenericSignature(abi),
+                  TypeOrigin::forUnspecified()))
+      return true;
+
+    return false;
+  }
+
+  bool visitVarDecl(VarDecl *api, VarDecl *abi) {
+    if (visitParentOfVarDecl(api, abi))
+      return true;
+    return false;
+  }
+
+  bool visitSubscriptDecl(SubscriptDecl *api, SubscriptDecl *abi) {
+    if (visitParentOfSubscriptDecl(api, abi))
+      return true;
+
+    if (checkParameterList(api->getIndices(), abi->getIndices(), api, abi))
+      return true;
+
+    return false;
+  }
+
+#undef UNSUPPORTED_DECL
+#undef PASSTHROUGH_DECL
+
+  // MARK: @abi checking - attributes
+
+  /// Are these attributes similar enough that they should be checked against
+  /// one another? At minimum this means they're of the same kind, but for some
+  /// attrs there are additional criteria.
+  bool canCompareAttrs(DeclAttribute *api, DeclAttribute *abi,
+                       Decl *apiDecl, Decl *abiDecl) {
+    if (api->getKind() != abi->getKind())
+      return false;
+
+    auto getAvailableDomain = [](Decl *D, DeclAttribute *A) {
+      return D->getSemanticAvailableAttr(cast<AvailableAttr>(A))->getDomain();
+    };
+
+    // Extra logic for specific attributes.
+    switch (api->getKind()) {
+    case DeclAttrKind::Expose:
+      return cast<ExposeAttr>(api)->getExposureKind()
+                  == cast<ExposeAttr>(abi)->getExposureKind();
+
+    case DeclAttrKind::Extern:
+      return cast<ExternAttr>(api)->getExternKind()
+                  == cast<ExternAttr>(abi)->getExternKind();
+
+    case DeclAttrKind::Available:
+      return getAvailableDomain(apiDecl, api)
+                  == getAvailableDomain(abiDecl, abi);
+      return true;
+
+    default:
+      break;
+    }
+
+    return true;
+  }
+
+  /// Check two attribute lists against one another.
+  /// 
+  /// This pairs up attributes which are sufficiently similar (as determined by
+  /// \c canCompareAttrs() ) and then checks them. Attributes which
+  /// have no counterpart are checked individually. 
+  bool checkAttrs(DeclAttributes api, DeclAttributes abi,
+                  Decl *apiDecl, Decl *abiDecl) {
+    bool didDiagnose = false;
+
+    // Collect all ABI attrs.
+    SmallVector<DeclAttribute*, 32> remainingABIDeclAttrs;
+    for (auto *abiDeclAttr : abi) {
+      remainingABIDeclAttrs.push_back(abiDeclAttr);
+    }
+
+    // Visit each API attr, pairing it with an ABI attr if possible.
+    // Note that this will visit even invalid attributes.
+    for (auto *apiDeclAttr : api) {
+      auto abiAttrIter = llvm::find_if(remainingABIDeclAttrs,
+                                        [&](DeclAttribute *abiDeclAttr) {
+        return abiDeclAttr && canCompareAttrs(apiDeclAttr, abiDeclAttr,
+                                              apiDecl, abiDecl);
+      });
+      DeclAttribute *abiDeclAttr = nullptr;
+      if (abiAttrIter != remainingABIDeclAttrs.end()) {
+        // Found a matching ABI attr. Claim and use it.
+        std::swap(abiDeclAttr, *abiAttrIter);
+      }
+      didDiagnose |= checkAttr(apiDeclAttr, abiDeclAttr, apiDecl, abiDecl);
+    }
+
+    // Visit leftover ABI attrs.
+    for (auto *abiDeclAttr : remainingABIDeclAttrs) {
+      if (abiDeclAttr)
+        didDiagnose |= checkAttr(nullptr, abiDeclAttr, apiDecl, abiDecl);
+    }
+    return didDiagnose;
+  }
+
+  /// Check a single attribute against its counterpart. If an attribute has no
+  /// counterpart, the counterpart may be \c nullptr ; either \p abi or \p abi
+  /// may be \c nullptr , but never both. 
+  bool checkAttr(DeclAttribute *api, DeclAttribute *abi,
+                 Decl *apiDecl, Decl *abiDecl) {
+    ASSERT(api || abi && "checkAttr() should have at least one attribute");
+
+    // If either attribute has already been diagnosed, don't check here.
+    if ((api && api->isInvalid()) || (abi && abi->isInvalid()))
+      return true;
+
+    auto kind = api ? api->getKind() : abi->getKind();
+    auto behaviors = DeclAttribute::getBehaviors(kind);
+
+    switch (behaviors & DeclAttribute::InABIAttrMask) {
+    case DeclAttribute::UnreachableInABIAttr:
+      ASSERT(abiAttr->canAppearOnDecl(apiDecl)
+                && "checking @abi on decl that can't have it???");
+      ASSERT(!abiAttr->canAppearOnDecl(apiDecl)
+                 && "unreachable-in-@abi attr on reachable decl???");
+
+      // If the asserts are disabled, fall through to no checking.
+      LLVM_FALLTHROUGH;
+
+    case DeclAttribute::UnconstrainedInABIAttr:
+      // No checking required.
+      return false;
+
+    case DeclAttribute::ForbiddenInABIAttr:
+      // Diagnose if ABI has attribute.
+      if (abi) {
+        // A shorthand `@available(foo 1, bar 2, *)` attribute gets parsed into
+        // several separate `AvailableAttr`s, each with the full range of the
+        // shorthand attribute. If we've already diagnosed one of them, don't
+        // diagnose the rest; otherwise, record that we've diagnosed this one.
+        if (isa<AvailableAttr>(abi) &&
+              !diagnosedAvailableAttrSourceLocs.insert(abi->getLocation()))
+          return true;
+
+        diagnoseAndRemoveAttr(abi, diag::attr_abi_forbidden_attr,
+                              abi->getAttrName(), abi->isDeclModifier());
+        return true;
+      }
+
+      return false;
+
+    case DeclAttribute::InferredInABIAttr:
+      if (!abi && api->canClone()) {
+        // Infer an identical attribute.
+        abi = api->clone(ctx);
+        abi->setImplicit(true);
+        abiDecl->getAttrs().add(abi);
+
+        if (ctx.LangOpts.EnableABIInferenceRemarks) {
+          SmallString<64> scratch;
+          auto abiAttrAsString = printAttr(abi, abiDecl, scratch);
+
+          abiDecl->diagnose(diag::abi_attr_inferred_attribute,
+                            abiAttrAsString, api->isDeclModifier());
+          noteAttrHere(api, apiDecl, /*isMatch=*/true);
+        }
+      }
+
+      // Other than the cloning behavior, Inferred behaves like Equivalent.
+      LLVM_FALLTHROUGH;
+
+    case DeclAttribute::EquivalentInABIAttr:
+      // Diagnose if API doesn't have attribute.
+      if (!api) {
+        diagnoseAndRemoveAttr(abi, diag::attr_abi_extra_attr,
+                              abi->getAttrName(), abi->isDeclModifier(),
+                              abi->isImplicit());
+        return true;
+      }
+
+      // Diagnose if ABI doesn't have attribute.
+      if (!abi) {
+        SmallString<64> scratch;
+        auto apiAttrAsString = printAttr(api, apiDecl, scratch,
+                                         /*toInsert=*/true);
+
+        ctx.Diags.diagnose(abiDecl, diag::attr_abi_missing_attr,
+                           api->getAttrName(), api->isDeclModifier())
+          .fixItInsert(abiDecl->getAttributeInsertionLoc(api->isDeclModifier()),
+                       apiAttrAsString);
+        noteAttrHere(api, apiDecl);
+        return true;
+      }
+
+      // Diagnose if two attributes are mismatched.
+      if (!api->isEquivalent(abi, apiDecl)) {
+        SmallString<64> scratch;
+        auto apiAttrAsString = printAttr(api, apiDecl, scratch);
+
+        ctx.Diags.diagnose(abi->getLocation(), diag::attr_abi_mismatched_attr,
+                           abi->getAttrName(), abi->isDeclModifier(),
+                           apiAttrAsString)
+          .fixItReplace(abi->getRangeWithAt(), apiAttrAsString);
+        noteAttrHere(api, apiDecl);
+        return true;
+      }
+
+      return false;
+    }
+
+    llvm_unreachable("unknown InABIAttrMask behavior");
+  }
+
+  // MARK: @abi checking - types
+
+  bool checkType(Type api, Type abi, SourceLoc apiLoc, SourceLoc abiLoc,
+                 GenericSignature apiSig, GenericSignature abiSig,
+                 TypeOrigin origin) {
+    if (!api.isNull() && !abi.isNull()) {
+      Type apiNorm = normalizeType(api->getReducedType(apiSig));
+      Type abiNorm = normalizeType(abi->getReducedType(abiSig));
+      if (apiNorm->isEqual(abiNorm)) {
+        return false;
+      }
+    }
+
+    ctx.Diags.diagnose(abiLoc, diag::attr_abi_mismatched_type,
+                       unsigned(origin.getKind()), origin.getDecl(), abi, api);
+    ctx.Diags.diagnose(apiLoc, diag::attr_abi_should_match_type_here);
+    return true;
+  }
+
+  /// Fold away details of \p original that do not affect the calling
+  /// conventions used for this type.
+  static Type normalizeType(Type original) {
+    return original.transformRec(&tryNormalizeOutermostType);
+  }
+
+  /// Fold away details of \p original that do not affect the calling
+  /// conventions used for this parameter. Does \em not fully normalize
+  /// \c original.getPlainType() , though it may slightly modify it.
+  /// Pass \c nullptr to \p forDecl for a parameter belonging to a closure.
+  static AnyFunctionType::Param
+  normalizeParam(const AnyFunctionType::Param &original, ValueDecl *forDecl) {
+    Type ty = original.getPlainType();
+    auto flags = original.getParameterFlags();
+
+    // We will smash away (non-parameter pack) variadics; turn the type into
+    // an array.
+    if (flags.isVariadic()) {
+      ty = original.getParameterType();
+    }
+
+    // Flatten ownership information down to consume/borrow/inout, which are the
+    // only distinctions that matter for calling conventions and memory
+    // management. This removes the distinction between e.g. `__owned` and
+    // `consuming`, or between the declaration's default parameter ownership
+    // convention and an explicit equivalent.
+    auto ownership = normalizeOwnership(flags.getOwnershipSpecifier(),
+                                        forDecl);
+
+    // Eliminate flags with no effect on the calling convention.
+    flags = flags
+      .withVariadic(false)
+      .withCompileTimeLiteral(false)
+      .withAutoClosure(false)
+      .withNonEphemeral(false)
+      .withIsolated(false)
+      .withSending(false)
+      .withOwnershipSpecifier(ownership);
+
+    return AnyFunctionType::Param(ty, Identifier(), flags, Identifier());
+  }
+
+  /// Folds away \p original to one of \c Consuming , \c Borrowing , or
+  /// \c InOut , which are the only parameter ownership behaviors relevant to ABI.
+  /// Pass \c nullptr to \p forDecl for a parameter belonging to a closure.
+  static ParamSpecifier normalizeOwnership(ParamSpecifier original,
+                                           ValueDecl *forDecl) {
+    switch (original) {
+    case ParamSpecifier::Default:
+      return getDefaultParamSpecifier(forDecl);
+      break;
+    case swift::ParamSpecifier::InOut:
+      return ParamSpecifier::InOut;
+      break;
+    case ParamSpecifier::Borrowing:
+    case ParamSpecifier::LegacyShared:
+      return ParamSpecifier::Borrowing;
+      break;
+    case swift::ParamSpecifier::Consuming:
+    case swift::ParamSpecifier::LegacyOwned:
+    case swift::ParamSpecifier::ImplicitlyCopyableConsuming:
+      return ParamSpecifier::Consuming;
+      break;
+    }
+  }
+
+  static GenericSignature normalizeGenericSignature(GenericSignature original) {
+    // FIXME: Are there other ABI-tolerable generic signature differences?
+    return original.withoutMarkerProtocols();
+  }
+
+  static
+  FunctionTypeIsolation normalizeIsolation(FunctionTypeIsolation original) {
+    // Isolation doesn't affect the ABI, except that `@isolated(any)` (a.k.a.
+    // `FunctionTypeIsolation::Kind::Erased`) has a different ABI from all the
+    // others. See docs/SIL/Types.td for details.
+    return original.isErased() ? FunctionTypeIsolation::forErased()
+                               : FunctionTypeIsolation::forNonIsolated();
+  }
+
+  static std::optional<Type> tryNormalizeOutermostType(TypeBase *original) {
+    // Function types: Eliminate anything that doesn't affect calling convention.
+    if (auto func = original->getAs<AnyFunctionType>()) {
+      // Ignore `@escaping`, `@Sendable`, `sending` on result, and most
+      // isolation; they have no ABI effect.
+      auto normalizedExt = func->getExtInfo().intoBuilder()
+        .withNoEscape(false)
+        .withSendable(false)
+        .withSendingResult(false)
+        .withIsolation(normalizeIsolation(func->getIsolation()))
+        .build();
+
+      // Ignore ignorable parts of parameters.
+      SmallVector<AnyFunctionType::Param, 8> normalizedParams;
+      for (auto param : func->getParams()) {
+        auto normalizedParam = normalizeParam(param, /*forDecl=*/nullptr);
+        normalizedParam = normalizedParam.withType(
+                                 normalizeType(normalizedParam.getPlainType()));
+        normalizedParams.push_back(normalizedParam);
+      }
+
+      if (isa<FunctionType>(func))
+        return FunctionType::get(normalizedParams,
+                                 normalizeType(func->getResult()),
+                                 normalizedExt);
+
+      ASSERT(isa<GenericFunctionType>(func));
+
+      // Ignore ignorable parts of the generic signature.
+      auto sig = original->getAs<GenericFunctionType>()->getGenericSignature();
+      return GenericFunctionType::get(normalizeGenericSignature(sig),
+                                      normalizedParams,
+                                      normalizeType(func->getResult()),
+                                      normalizedExt);
+    }
+
+    // Protocol-related types: Remove marker protocols.
+    if (auto comp = original->getAs<ProtocolCompositionType>()) {
+      auto normalized = comp->withoutMarkerProtocols();
+      if (!normalized->isEqual(comp))
+        return normalized;
+    }
+
+    if (auto proto = original->getAs<ProtocolType>()) {
+      if (proto->getDecl()->isMarkerProtocol())
+        return proto->getASTContext().TheAnyType;
+    }
+
+    if (auto existential = original->getAs<ExistentialType>()) {
+      // Pull out the constraint and see how it'll normalize.
+      auto normConstraint = normalizeType(existential->getConstraintType());
+
+      // If the constraint is no longer existential, pull it out of the type.
+      if (!normConstraint->isExistentialType())
+        return normConstraint;
+    }
+
+    // Tuples: Remove labels.
+    if (auto tuple = original->getAs<TupleType>()) {
+      bool needsNormalization = false;
+      SmallVector<TupleTypeElt, 8> unlabeledElements;
+
+      for (auto elem : tuple->getElements()) {
+        needsNormalization |= !elem.getName().empty();
+        unlabeledElements.push_back(elem.getWithoutName());
+      }
+
+      if (!needsNormalization)
+        return tuple;
+
+      return TupleType::get(unlabeledElements, tuple->getASTContext());
+    }
+
+    // TODO: Allow Optional/non-Optional variance when ABI-compatible?
+    // TODO: Allow variance in exact class of object?
+
+    return std::nullopt;
+  }
+};
+
+void checkABIAttrPBD(PatternBindingDecl *APBD, VarDecl *VD) {
+  auto &diags = VD->getASTContext().Diags;
+  auto PBD = VD->getParentPatternBinding();
+
+  // To make sure we only diagnose this stuff once, check that VD is the first
+  // anchoring variable in the PBD.
+  bool isFirstAnchor = false;
+  for (auto i : range(PBD->getNumPatternEntries())) {
+    auto anchorVD = PBD->getAnchoringVarDecl(i);
+    if (anchorVD) {
+      isFirstAnchor = (anchorVD == VD);
+      break;
+    }
+  }
+
+  if (!isFirstAnchor)
+    return;
+
+  // Check that the PBDs have the same number of patterns.
+  if (PBD->getNumPatternEntries() < APBD->getNumPatternEntries()) {
+    diags.diagnose(APBD->getPattern(PBD->getNumPatternEntries())->getLoc(),
+                   diag::attr_abi_mismatched_pbd_size, /*abiHasExtra=*/false);
+    return;
+  }
+  if (PBD->getNumPatternEntries() > APBD->getNumPatternEntries()) {
+    diags.diagnose(PBD->getPattern(APBD->getNumPatternEntries())->getLoc(),
+                   diag::attr_abi_mismatched_pbd_size, /*abiHasExtra=*/true);
+    return;
+  }
+
+  // Check that each pattern has the same number of variables.
+  for (auto i : range(PBD->getNumPatternEntries())) {
+    SmallVector<VarDecl *, 8> VDs;
+    SmallVector<VarDecl *, 8> AVDs;
+
+    PBD->getPattern(i)->collectVariables(VDs);
+    APBD->getPattern(i)->collectVariables(AVDs);
+
+    if (VDs.size() < AVDs.size()) {
+      for (auto AVD : drop_begin(AVDs, VDs.size())) {
+        AVD->diagnose(diag::attr_abi_mismatched_var,
+                      AVD, /*isABI=*/true);
+      }
+    }
+    else if (VDs.size() > AVDs.size()) {
+      for (auto VD : drop_begin(VDs, AVDs.size())) {
+        VD->diagnose(diag::attr_abi_mismatched_var,
+                     VD, /*isABI=*/false);
+      }
+    }
+  }
+}
+} // end anonymous namespace
+
+void TypeChecker::checkDeclABIAttribute(Decl *D, ABIAttr *attr) {
+  Decl *AD = attr->abiDecl;
+  if (isa<VarDecl>(D) && isa<PatternBindingDecl>(AD)) {
+    auto VD = cast<VarDecl>(D);
+    auto APBD = cast<PatternBindingDecl>(AD);
+
+    // Diagnose dissimilar PBD structures.
+    checkABIAttrPBD(APBD, VD);
+
+    // Do the rest of this checking on the corresponding VarDecl, not the
+    // PBD that's actually in the attribute. Note that `AD` will become null
+    // if they're too dissimilar to match up.
+    AD = APBD->getVarAtSimilarStructuralPosition(VD);
+  }
+  // TODO: EnumElementDecl?
+
+  if (!AD)
+    return;
+
+  // Check the ABI decl and bail if there was a problem with it.
+  TypeChecker::typeCheckDecl(AD);
+  if (AD->isInvalid())
+    return;
+
+  // Apply more precise checks.
+  ABIDeclChecker(D->getASTContext(), D, attr).check(D, AD);
+}
diff --git a/lib/Sema/TypeCheckAvailability.cpp b/lib/Sema/TypeCheckAvailability.cpp
index 32c323d838f86..1dabd633b9a14 100644
--- a/lib/Sema/TypeCheckAvailability.cpp
+++ b/lib/Sema/TypeCheckAvailability.cpp
@@ -480,6 +480,12 @@ static const Decl *relatedDeclForAvailabilityFixit(const Decl *D) {
     D = accessor->getStorage();
   }
 
+  auto abiRole = ABIRoleInfo(D);
+  if (!abiRole.providesAPI() && abiRole.getCounterpart()) {
+    // ABI-only decls can't have @available attributes of their own.
+    D = abiRole.getCounterpart();
+  }
+
   return D->getAbstractSyntaxDeclForAttributes();
 }
 
@@ -645,7 +651,7 @@ static void fixAvailabilityForDecl(
   // syntax to suggest the Fix-It may differ from the declaration to which
   // we attach availability attributes in the abstract syntax tree during
   // parsing.
-  const Decl *ConcDecl = D->getConcreteSyntaxDeclForAttributes();
+  const Decl *ConcDecl = relatedDeclForAvailabilityFixit(D);
 
   // To avoid exposing the pattern binding declaration to the user, get the
   // descriptive kind from one of the VarDecls.
diff --git a/lib/Sema/TypeCheckDecl.cpp b/lib/Sema/TypeCheckDecl.cpp
index 228c6bc6f0c74..0b6f5c360ba11 100644
--- a/lib/Sema/TypeCheckDecl.cpp
+++ b/lib/Sema/TypeCheckDecl.cpp
@@ -938,6 +938,11 @@ IsStaticRequest::evaluate(Evaluator &evaluator, FuncDecl *decl) const {
 
 bool
 IsDynamicRequest::evaluate(Evaluator &evaluator, ValueDecl *decl) const {
+  // ABI-only decls get this from their API decl.
+  auto abiRole = ABIRoleInfo(decl);
+  if (!abiRole.providesAPI() && abiRole.getCounterpart())
+    return abiRole.getCounterpart()->isDynamic();
+
   // If we can't infer dynamic here, don't.
   if (!DeclAttribute::canAttributeAppearOnDecl(DeclAttrKind::Dynamic, decl))
     return false;
diff --git a/lib/Sema/TypeCheckDeclObjC.cpp b/lib/Sema/TypeCheckDeclObjC.cpp
index 3bf4ff5d53f6b..ec38710102b22 100644
--- a/lib/Sema/TypeCheckDeclObjC.cpp
+++ b/lib/Sema/TypeCheckDeclObjC.cpp
@@ -650,6 +650,11 @@ bool swift::isRepresentableInObjC(
     const AbstractFunctionDecl *AFD, ObjCReason Reason,
     std::optional<ForeignAsyncConvention> &asyncConvention,
     std::optional<ForeignErrorConvention> &errorConvention) {
+  auto abiRole = ABIRoleInfo(AFD);
+  if (!abiRole.providesAPI() && abiRole.getCounterpart())
+    return isRepresentableInObjC(abiRole.getCounterpart(), Reason,
+                                 asyncConvention, errorConvention);
+
   // Clear out the async and error conventions. They will be added later if
   // needed.
   asyncConvention = std::nullopt;
@@ -1104,6 +1109,10 @@ bool swift::isRepresentableInObjC(const VarDecl *VD, ObjCReason Reason) {
   if (VD->isInvalid())
     return false;
 
+  auto abiRole = ABIRoleInfo(VD);
+  if (!abiRole.providesAPI() && abiRole.getCounterpart())
+    return isRepresentableInObjC(abiRole.getCounterpart(), Reason);
+
   Type T = VD->getDeclContext()->mapTypeIntoContext(VD->getInterfaceType());
   if (auto *RST = T->getAs<ReferenceStorageType>()) {
     // In-memory layout of @weak and @unowned does not correspond to anything
@@ -1160,6 +1169,10 @@ bool swift::isRepresentableInObjC(const VarDecl *VD, ObjCReason Reason) {
 
 bool swift::isRepresentableInObjC(const SubscriptDecl *SD, ObjCReason Reason) {
   // If you change this function, you must add or modify a test in PrintAsClang.
+  auto abiRole = ABIRoleInfo(SD);
+  if (!abiRole.providesAPI() && abiRole.getCounterpart())
+    return isRepresentableInObjC(abiRole.getCounterpart(), Reason);
+
   ASTContext &ctx = SD->getASTContext();
   DiagnosticStateRAII diagState(ctx.Diags);
   auto behavior = behaviorLimitForObjCReason(Reason, ctx);
@@ -1770,6 +1783,11 @@ static void markAsObjC(ValueDecl *D, ObjCReason reason,
 bool IsObjCRequest::evaluate(Evaluator &evaluator, ValueDecl *VD) const {
   DiagnosticStateRAII diagState(VD->getASTContext().Diags);
 
+  // ABI-only decls inherit objc-ness from their API.
+  auto abiRole = ABIRoleInfo(VD);
+  if (!abiRole.providesAPI() && abiRole.getCounterpart())
+    return abiRole.getCounterpart()->isObjC();
+
   // Access notes may add attributes that affect this calculus.
   TypeChecker::applyAccessNote(VD);
 
@@ -4115,6 +4133,9 @@ class ObjCImplementationChecker {
 
 evaluator::SideEffect TypeCheckObjCImplementationRequest::
 evaluate(Evaluator &evaluator, Decl *D) const {
+  if (!ABIRoleInfo(D).providesAPI())
+    return evaluator::SideEffect();
+
   PrettyStackTraceDecl trace("checking member implementations of", D);
 
   // FIXME: Because we check extension-by-extension, candidates and requirements
diff --git a/lib/Sema/TypeCheckDeclOverride.cpp b/lib/Sema/TypeCheckDeclOverride.cpp
index 4af7efdfcded1..85a113951a1e4 100644
--- a/lib/Sema/TypeCheckDeclOverride.cpp
+++ b/lib/Sema/TypeCheckDeclOverride.cpp
@@ -2098,7 +2098,8 @@ static bool checkSingleOverride(ValueDecl *override, ValueDecl *base) {
   if (!override->getAttrs().hasAttribute<OverrideAttr>() &&
       overrideRequiresKeyword(base) != OverrideRequiresKeyword::Never &&
       !override->isImplicit() &&
-      override->getDeclContext()->getParentSourceFile()) {
+      override->getDeclContext()->getParentSourceFile() &&
+      ABIRoleInfo(override).providesAPI()) {
     auto theDiag =
       overrideRequiresKeyword(base) == OverrideRequiresKeyword::Always
         ? diag::missing_override
@@ -2513,6 +2514,19 @@ computeOverriddenDecls(ValueDecl *decl, bool ignoreMissingImports) {
 
 llvm::TinyPtrVector<ValueDecl *>
 OverriddenDeclsRequest::evaluate(Evaluator &evaluator, ValueDecl *decl) const {
+  auto abiRole = ABIRoleInfo(decl);
+  if (!abiRole.providesAPI() && abiRole.getCounterpart()) {
+    auto apiOverriddenDecls = abiRole.getCounterpart()->getOverriddenDecls();
+
+    TinyPtrVector<ValueDecl *> abiOverriddenDecls;
+    for (auto apiOverriddenDecl : apiOverriddenDecls) {
+      auto abiOverriddenDecl = ABIRoleInfo(apiOverriddenDecl).getCounterpart();
+      abiOverriddenDecls.push_back(abiOverriddenDecl);
+    }
+
+    return abiOverriddenDecls;
+  }
+
   auto &ctx = decl->getASTContext();
   auto overridden = computeOverriddenDecls(decl, false);
 
diff --git a/lib/Sema/TypeCheckDeclPrimary.cpp b/lib/Sema/TypeCheckDeclPrimary.cpp
index 5664aff3f48f3..033274012edcf 100644
--- a/lib/Sema/TypeCheckDeclPrimary.cpp
+++ b/lib/Sema/TypeCheckDeclPrimary.cpp
@@ -2173,6 +2173,10 @@ void swift::diagnoseAttrsAddedByAccessNote(SourceFile &SF) {
 
 evaluator::SideEffect
 ApplyAccessNoteRequest::evaluate(Evaluator &evaluator, ValueDecl *VD) const {
+  // Access notes don't apply to ABI-only attributes.
+  if (!ABIRoleInfo(VD).providesAPI())
+    return {};
+
   AccessNotesFile &notes = VD->getModuleContext()->getAccessNotes();
   if (auto note = notes.lookup(VD))
     applyAccessNote(VD, *note.get(), notes);
diff --git a/lib/Sema/TypeCheckProtocol.cpp b/lib/Sema/TypeCheckProtocol.cpp
index 114eea73ddd5e..3df6c0611e531 100644
--- a/lib/Sema/TypeCheckProtocol.cpp
+++ b/lib/Sema/TypeCheckProtocol.cpp
@@ -6279,11 +6279,7 @@ static bool hasExplicitObjCName(ClassDecl *classDecl) {
   if (classDecl->getAttrs().hasAttribute<ObjCRuntimeNameAttr>())
     return true;
 
-  auto objcAttr = classDecl->getAttrs().getAttribute<ObjCAttr>();
-  if (!objcAttr)
-    return false;
-
-  return objcAttr->hasName() && !objcAttr->isNameImplicit();
+  return classDecl->getExplicitObjCName().has_value();
 }
 
 /// Check if the name of a class might be unstable, and if so, emit a
diff --git a/lib/Sema/TypeCheckStorage.cpp b/lib/Sema/TypeCheckStorage.cpp
index 4a6af64e8148d..ea7cb6fd2bd87 100644
--- a/lib/Sema/TypeCheckStorage.cpp
+++ b/lib/Sema/TypeCheckStorage.cpp
@@ -3733,6 +3733,11 @@ static StorageImplInfo classifyWithHasStorageAttr(VarDecl *var) {
 
 bool HasStorageRequest::evaluate(Evaluator &evaluator,
                                  AbstractStorageDecl *storage) const {
+  // ABI decl inherits this from API.
+  auto abiRole = ABIRoleInfo(storage);
+  if (!abiRole.providesAPI() && abiRole.getCounterpart())
+    return abiRole.getCounterpart()->hasStorage();
+
   // Parameters are always stored.
   if (isa<ParamDecl>(storage))
     return true;
@@ -3818,7 +3823,11 @@ void HasStorageRequest::cacheResult(bool hasStorage) const {
     return;
   
   if (auto varDecl = dyn_cast<VarDecl>(decl)) {
-    if (hasStorage && !varDecl->getAttrs().hasAttribute<HasStorageAttr>())
+    auto abiRole = ABIRoleInfo(varDecl);
+    bool abiOnly = !abiRole.providesAPI() && abiRole.getCounterpart();
+
+    if (hasStorage && !abiOnly &&
+          !varDecl->getAttrs().hasAttribute<HasStorageAttr>())
       varDecl->getAttrs().add(new (varDecl->getASTContext())
                               HasStorageAttr(/*isImplicit=*/true));
   }
diff --git a/lib/Sema/TypeChecker.h b/lib/Sema/TypeChecker.h
index e79d4247f1a9e..cb14ea0bf1b93 100644
--- a/lib/Sema/TypeChecker.h
+++ b/lib/Sema/TypeChecker.h
@@ -506,6 +506,7 @@ void typeCheckDecl(Decl *D);
 
 void addImplicitDynamicAttribute(Decl *D);
 void checkDeclAttributes(Decl *D);
+void checkDeclABIAttribute(Decl *apiDecl, ABIAttr *abiAttr);
 void checkClosureAttributes(ClosureExpr *closure);
 void checkParameterList(ParameterList *params, DeclContext *owner);
 
diff --git a/lib/Serialization/Deserialization.cpp b/lib/Serialization/Deserialization.cpp
index d464f27eac5f3..11541a24eb2c1 100644
--- a/lib/Serialization/Deserialization.cpp
+++ b/lib/Serialization/Deserialization.cpp
@@ -4025,7 +4025,8 @@ class DeclDeserializer {
       AddAttribute(new (ctx) OverrideAttr(SourceLoc()));
 
     // Add the @_hasStorage attribute if this var has storage.
-    if (var->hasStorage())
+    // (Unless it's an ABI-only decl--they shouldn't have a HasStorageAttr.)
+    if (var->hasStorage() && ABIDeclCounterpartID == 0)
       AddAttribute(new (ctx) HasStorageAttr(/*isImplicit:*/true));
 
     {
diff --git a/test/ASTGen/attrs.swift b/test/ASTGen/attrs.swift
index d55a408c48d46..307dd7770c3b1 100644
--- a/test/ASTGen/attrs.swift
+++ b/test/ASTGen/attrs.swift
@@ -94,7 +94,7 @@ struct S4 {}
 @implementation extension ObjCClass1 {} // expected-error {{cannot find type 'ObjCClass1' in scope}}
 @implementation(Category) extension ObjCClass1 {} // expected-error {{cannot find type 'ObjCClass1' in scope}}
 
-@abi(func fn_abi()) // expected-error {{cannot give global function 'fn' the ABI of a global function with a different number of low-level parameters}}
+@abi(func fn_abi()) // expected-error {{cannot give global function 'fn' the ABI of a global function with a different number of parameters}}
 func fn(_: Int) {}
 
 @_alignment(8) struct AnyAlignment {}
diff --git a/test/IRGen/asmname.swift b/test/IRGen/asmname.swift
index b86cc485723c1..50d40110adfb7 100644
--- a/test/IRGen/asmname.swift
+++ b/test/IRGen/asmname.swift
@@ -88,19 +88,24 @@ extension X {
 // CHECK: define{{( dllexport)?}}{{( protected)?}} swiftcc void @"$s7asmname21abi_ABIAttrPublic_vars5Int64Vvs"
 // CHECK: define hidden swiftcc i64 @"$s7asmname23abi_ABIAttrInternal_vars5Int64Vvg"
 // CHECK: define hidden swiftcc void @"$s7asmname23abi_ABIAttrInternal_vars5Int64Vvs"
-// CHECK: define internal swiftcc i64 @"$s7asmname22abi_ABIAttrPrivate_vars5Int64Vvg"
-// CHECK: define internal swiftcc void @"$s7asmname22abi_ABIAttrPrivate_vars5Int64Vvs"
+// CHECK: define internal swiftcc i64 @"$s7asmname22abi_ABIAttrPrivate_var33_{{[0-9A-F]+}}LLs5Int64Vvg"
+// CHECK: define internal swiftcc void @"$s7asmname22abi_ABIAttrPrivate_var33_{{[0-9A-F]+}}LLs5Int64Vvs"
 
-@abi(public func abi_ABIAttrGenericNonSendableToSendable<T>(_ value: T) -> T)
+@abi(func abi_ABIAttrGenericNonSendableToSendable<T>(_ value: T) -> T)
 public func api_ABIAttrGenericNonSendableToSendable<T: Sendable>(_ value: T) -> T { return value }
 // CHECK: define{{( dllexport)?}}{{( protected)?}} swiftcc void @"$s7asmname031abi_ABIAttrGenericNonSendableToF0yxxlF"
 // NEGATIVE-NOT: @"$s7asmname031abi_ABIAttrGenericNonSendableToF0yxxs0F0RzlF"
 
 // Similar to hack applied to `AsyncStream.init(unfolding:onCancel:)`
-@abi(public func abi_ABIAttrPreconcurrencyToNotPreconcurrency(_ c1: () -> Void, _ c2: @Sendable () -> Void))
+@abi(func abi_ABIAttrPreconcurrencyToNotPreconcurrency(_ c1: () -> Void, _ c2: @Sendable () -> Void))
 @preconcurrency public func api_ABIAttrPreconcurrencyToNotPreconcurrency(_ c1: () -> Void, _ c2: @Sendable () -> Void) {}
 // CHECK: define{{( dllexport)?}}{{( protected)?}} swiftcc void @"$s7asmname030abi_ABIAttrPreconcurrencyToNotD0yyyyXE_yyYbXEtF"
 
+@abi(var abi_ABIAttrVarEffects: Int)
+public var api_ABIAttrVarEffects: Int { get async throws { fatalError() } }
+// CHECK: define{{( dllexport)?}}{{( protected)?}} swifttailcc void @"$s7asmname21abi_ABIAttrVarEffectsSivg"
+// CHECK: define internal swifttailcc void @"$s7asmname21abi_ABIAttrVarEffectsSivgTY0_"
+
 extension X {
 // CHECK: define{{( dllexport)?}}{{( protected)?}} swiftcc { i64, i8 } @"$s7asmname1XO8abi_blahACs5Int64V_tcfC"
   @abi(init(abi_blah: Int64))
diff --git a/test/ModuleInterface/attrs.swift b/test/ModuleInterface/attrs.swift
index cbda5e5ca8e01..effaa641cc9b7 100644
--- a/test/ModuleInterface/attrs.swift
+++ b/test/ModuleInterface/attrs.swift
@@ -1,10 +1,13 @@
 // RUN: %target-swift-emit-module-interface(%t.swiftinterface) %s -module-name attrs \
+// RUN:  -emit-private-module-interface-path %t.private.swiftinterface \
 // RUN:  -enable-experimental-feature ABIAttribute \
 // RUN:  -enable-experimental-feature ExecutionAttribute
 
 // RUN: %target-swift-typecheck-module-from-interface(%t.swiftinterface) -module-name attrs
+// RUN: %target-swift-typecheck-module-from-interface(%t.private.swiftinterface) -module-name attrs
 
-// RUN: %FileCheck %s --input-file %t.swiftinterface
+// RUN: %FileCheck %s --check-prefixes CHECK,PUBLIC-CHECK --input-file %t.swiftinterface
+// RUN: %FileCheck %s --check-prefixes CHECK,PRIVATE-CHECK --input-file %t.private.swiftinterface
 
 // REQUIRES: swift_feature_ABIAttribute
 // REQUIRES: swift_feature_ExecutionAttribute
@@ -35,26 +38,55 @@ internal func __specialize_someGenericFunction<T>(_ t: T) -> Int {
     fatalError("don't call")
 }
 
-@abi(public func __abi__abiAttrOnFunction(param: Int))
+@abi(func __abi__abiAttrOnFunction(param: Int))
 public func abiAttrOnFunction(param: Int) {}
 // CHECK: #if {{.*}} $ABIAttribute
-// CHECK: @abi(public func __abi__abiAttrOnFunction(param: Swift.Int))
+// CHECK: @abi(func __abi__abiAttrOnFunction(param: Swift.Int))
 // CHECK: public func abiAttrOnFunction(param: Swift.Int)
 // CHECK: #else
 // CHECK: @_silgen_name("$s5attrs07__abi__B14AttrOnFunction5paramySi_tF")
 // CHECK: public func abiAttrOnFunction(param: Swift.Int)
 // CHECK: #endif
 
-@abi(public let __abi__abiAttrOnVar: Int)
+@abi(let __abi__abiAttrOnVar: Int)
 public var abiAttrOnVar: Int = 42
 // CHECK: #if {{.*}} $ABIAttribute
-// CHECK: @abi(public var __abi__abiAttrOnVar: Swift.Int)
+// CHECK: @abi(var __abi__abiAttrOnVar: Swift.Int)
 // CHECK: public var abiAttrOnVar: Swift.Int
 // CHECK: #else
 // CHECK: @available(*, unavailable, message: "this compiler cannot match the ABI specified by the @abi attribute")
 // CHECK: public var abiAttrOnVar: Swift.Int
 // CHECK: #endif
 
+public struct MutatingTest {
+  // CHECK: #if {{.*}} $ABIAttribute
+  // CHECK: @abi(mutating func abiMutFunc())
+  // CHECK: public mutating func abiMutFunc()
+  // CHECK: #else
+  // CHECK: @_silgen_name("$s5attrs12MutatingTestV10abiMutFuncyyF")
+  // CHECK: public mutating func abiMutFunc()
+  // CHECK: #endif
+  @abi(mutating func abiMutFunc())
+  public mutating func abiMutFunc() {}
+}
+
+// PUBLIC-CHECK-NOT: #if {{.*}} $ABIAttribute
+// PUBLIC-CHECK-NOT: @abi(func abiSpiFunc())
+// PUBLIC-CHECK-NOT: public func abiSpiFunc()
+// PUBLIC-CHECK-NOT: #else
+// PUBLIC-CHECK-NOT: @_silgen_name("$s5attrs10abiSpiFuncyyF")
+// PUBLIC-CHECK-NOT: public func abiSpiFunc()
+// PUBLIC-CHECK-NOT: #endif
+// PRIVATE-CHECK: #if {{.*}} $ABIAttribute
+// PRIVATE-CHECK: @abi(func abiSpiFunc())
+// PRIVATE-CHECK: public func abiSpiFunc()
+// PRIVATE-CHECK: #else
+// PRIVATE-CHECK: @_silgen_name("$s5attrs10abiSpiFuncyyF")
+// PRIVATE-CHECK: public func abiSpiFunc()
+// PRIVATE-CHECK: #endif
+@abi(func abiSpiFunc())
+@_spi(spiGroup) public func abiSpiFunc() {}
+
 @execution(concurrent)
 public func testExecutionConcurrent() async {}
 // CHECK: @execution(concurrent) public func testExecutionConcurrent() async
diff --git a/test/ModuleInterface/attrs_objc.swift b/test/ModuleInterface/attrs_objc.swift
new file mode 100644
index 0000000000000..37cb78318be05
--- /dev/null
+++ b/test/ModuleInterface/attrs_objc.swift
@@ -0,0 +1,45 @@
+// RUN: %target-swift-emit-module-interface(%t.swiftinterface) %s \
+// RUN:  -enable-objc-interop -module-name attrs_objc \
+// RUN:  -enable-experimental-feature ABIAttribute
+
+// RUN: %target-swift-typecheck-module-from-interface(%t.swiftinterface) -module-name attrs_objc
+
+// RUN: %FileCheck %s --input-file %t.swiftinterface
+
+// REQUIRES: objc_interop
+// REQUIRES: swift_feature_ABIAttribute
+
+import Foundation
+
+@objcMembers
+public class ObjCTest: NSObject {
+  // CHECK: #if {{.*}} $ABIAttribute
+  // CHECK: @abi(func abiObjCFunc())
+  // CHECK: @objc public func abiObjCFunc()
+  // CHECK: #else
+  // CHECK: @_silgen_name("$s10attrs_objc8ObjCTestC03abiC5CFuncyyF")
+  // CHECK: @objc public func abiObjCFunc()
+  // CHECK: #endif
+  @abi(func abiObjCFunc())
+  @objc public func abiObjCFunc() {}
+
+  // CHECK: #if {{.*}} $ABIAttribute
+  // CHECK: @abi(func abiImplicitObjCFunc())
+  // CHECK: @objc public func abiImplicitObjCFunc()
+  // CHECK: #else
+  // CHECK: @_silgen_name("$s10attrs_objc8ObjCTestC011abiImplicitC5CFuncyyF")
+  // CHECK: @objc public func abiImplicitObjCFunc()
+  // CHECK: #endif
+  @abi(func abiImplicitObjCFunc())
+  public func abiImplicitObjCFunc() {}
+
+  // CHECK: #if {{.*}} $ABIAttribute
+  // CHECK: @abi(func abiIBActionFunc(_: Any))
+  // CHECK: @objc @IBAction @_Concurrency.MainActor @preconcurrency public func abiIBActionFunc(_: Any)
+  // CHECK: #else
+  // CHECK: @_silgen_name("$s10attrs_objc8ObjCTestC15abiIBActionFuncyyypF")
+  // CHECK: @objc @IBAction @_Concurrency.MainActor @preconcurrency public func abiIBActionFunc(_: Any)
+  // CHECK: #endif
+  @abi(func abiIBActionFunc(_: Any))
+  @IBAction public func abiIBActionFunc(_: Any) {}
+}
diff --git a/test/attr/Inputs/attr_abi.h b/test/attr/Inputs/attr_abi.h
new file mode 100644
index 0000000000000..580cdfb6b20c2
--- /dev/null
+++ b/test/attr/Inputs/attr_abi.h
@@ -0,0 +1,3 @@
+void implementation1(void);
+void implementation2(void);
+void implementation3(void);
diff --git a/test/attr/attr_abi.swift b/test/attr/attr_abi.swift
index a0f8a8e21ea8b..1409d75bf0fdf 100644
--- a/test/attr/attr_abi.swift
+++ b/test/attr/attr_abi.swift
@@ -1,7 +1,21 @@
-// RUN: %target-typecheck-verify-swift -enable-experimental-feature Extern -enable-experimental-feature ABIAttribute -parse-as-library
+// RUN: %target-typecheck-verify-swift -enable-experimental-feature Extern -enable-experimental-feature ABIAttribute -enable-experimental-feature AddressableParameters -enable-experimental-feature NoImplicitCopy -enable-experimental-feature SymbolLinkageMarkers -enable-experimental-feature StrictMemorySafety -enable-experimental-feature LifetimeDependence -enable-experimental-feature CImplementation -enable-experimental-feature ExecutionAttribute -import-bridging-header %S/Inputs/attr_abi.h -parse-as-library -Rabi-inference -debugger-support
 
 // REQUIRES: swift_feature_ABIAttribute
+// REQUIRES: swift_feature_AddressableParameters
+// REQUIRES: swift_feature_CImplementation
+// REQUIRES: swift_feature_ExecutionAttribute
 // REQUIRES: swift_feature_Extern
+// REQUIRES: swift_feature_LifetimeDependence
+// REQUIRES: swift_feature_NoImplicitCopy
+// REQUIRES: swift_feature_StrictMemorySafety
+// REQUIRES: swift_feature_SymbolLinkageMarkers
+
+import _Differentiation
+
+import Distributed
+
+@available(SwiftStdlib 5.7, *)
+typealias DefaultDistributedActorSystem = LocalTestingDistributedActorSystem
 
 //
 // Same-kind checking
@@ -19,6 +33,23 @@ var funcForVar: Int = 0
 @abi(var varForFunc_abi: Int) // expected-error {{cannot give global function 'varForFunc()' the ABI of a pattern binding}}
 func varForFunc() {}
 
+struct SameKind {
+  @abi(subscript(sub1 _: Int) -> Int)
+  subscript(sub1 _: Int) -> Int { 0 }
+
+  @abi(func sub2(_: Int) -> Int) // expected-error {{cannot give subscript 'subscript(sub2:)' the ABI of a instance method}}
+  subscript(sub2 _: Int) -> Int { 0 }
+
+  @abi(subscript(sub3 _: Int) -> Int) // expected-error {{cannot give instance method 'sub3' the ABI of a subscript}}
+  func sub3(_: Int) -> Int { 0 }
+
+  @abi(var sub4: Int) // expected-error {{cannot give subscript 'subscript(sub4:)' the ABI of a pattern binding}}
+  subscript(sub4 _: Int) -> Int { 0 }
+
+  @abi(subscript(sub4 _: Int) -> Int) // expected-error {{cannot give property 'sub4' the ABI of a subscript}}
+  var sub4: Int { 0 }
+}
+
 //
 // Function arity checking
 //
@@ -26,10 +57,10 @@ func varForFunc() {}
 @abi(func param00_generic00() -> Int)
 func param00_generic00() -> Int { fatalError() }
 
-@abi(func param10_generic00(_: Int) -> Int) // expected-error {{cannot give global function 'param10_generic00()' the ABI of a global function with a different number of low-level parameters}}
+@abi(func param10_generic00(_: Int) -> Int) // expected-error {{cannot give global function 'param10_generic00()' the ABI of a global function with a different number of parameters}}
 func param10_generic00() -> Int { fatalError() }
 
-@abi(func param01_generic00() -> Int) // expected-error {{cannot give global function 'param01_generic00' the ABI of a global function with a different number of low-level parameters}}
+@abi(func param01_generic00() -> Int) // expected-error {{cannot give global function 'param01_generic00' the ABI of a global function with a different number of parameters}}
 func param01_generic00(_: Int) -> Int { fatalError() }
 
 @abi(func param11_generic00(_: Int) -> Int)
@@ -37,30 +68,30 @@ func param11_generic00(_: Int) -> Int { fatalError() }
 
 
 
-@abi(func param00_generic10<T>() -> T) // expected-error {{cannot give global function 'param00_generic10()' the ABI of a global function with a different number of low-level parameters}}
+@abi(func param00_generic10<T>() -> T) // expected-error {{declaration in '@abi' should not have generic signature}}
 func param00_generic10() -> Int { fatalError() }
 
-@abi(func param10_generic10<T>(_: Int) -> T) // expected-error {{cannot give global function 'param10_generic10()' the ABI of a global function with a different number of low-level parameters}}
+@abi(func param10_generic10<T>(_: Int) -> T) // expected-error {{declaration in '@abi' should not have generic signature}}
 func param10_generic10() -> Int { fatalError() }
 
-@abi(func param01_generic10<T>() -> T)
+@abi(func param01_generic10<T>() -> T) // expected-error {{declaration in '@abi' should not have generic signature because 'param01_generic10' is not generic}}
 func param01_generic10(_: Int) -> Int { fatalError() }
 
-@abi(func param11_generic10<T>(_: Int) -> T) // expected-error {{cannot give global function 'param11_generic10' the ABI of a global function with a different number of low-level parameters}}
+@abi(func param11_generic10<T>(_: Int) -> T) // expected-error {{declaration in '@abi' should not have generic signature because 'param11_generic10' is not generic}}
 func param11_generic10(_: Int) -> Int { fatalError() }
 
 
 
-@abi(func param00_generic01() -> Int) // expected-error {{cannot give global function 'param00_generic01()' the ABI of a global function with a different number of low-level parameters}}
+@abi(func param00_generic01() -> Int) // expected-error {{declaration in '@abi' should have generic signature compatible with '<T where T : Copyable, T : Escapable>'}}
 func param00_generic01<T>() -> T { fatalError() }
 
-@abi(func param10_generic01(_: Int) -> Int)
+@abi(func param10_generic01(_: Int) -> Int) // expected-error {{declaration in '@abi' should have generic signature compatible with '<T where T : Copyable, T : Escapable>'}}
 func param10_generic01<T>() -> T { fatalError() }
 
-@abi(func param01_generic01() -> Int) // expected-error {{cannot give global function 'param01_generic01' the ABI of a global function with a different number of low-level parameters}}
+@abi(func param01_generic01() -> Int) // expected-error {{declaration in '@abi' should have generic signature compatible with '<T where T : Copyable, T : Escapable>'}}
 func param01_generic01<T>(_: Int) -> T { fatalError() }
 
-@abi(func param11_generic01(_: Int) -> Int) // expected-error {{cannot give global function 'param11_generic01' the ABI of a global function with a different number of low-level parameters}}
+@abi(func param11_generic01(_: Int) -> Int) // expected-error {{declaration in '@abi' should have generic signature compatible with '<T where T : Copyable, T : Escapable>'}}
 func param11_generic01<T>(_: Int) -> T { fatalError() }
 
 
@@ -68,19 +99,73 @@ func param11_generic01<T>(_: Int) -> T { fatalError() }
 @abi(func param00_generic11<T>() -> T)
 func param00_generic11<T>() -> T { fatalError() }
 
-@abi(func param10_generic11<T>(_: Int) -> T) // expected-error {{cannot give global function 'param10_generic11()' the ABI of a global function with a different number of low-level parameters}}
+@abi(func param10_generic11<T>(_: Int) -> T) // expected-error {{cannot give global function 'param10_generic11()' the ABI of a global function with a different number of parameters}}
 func param10_generic11<T>() -> T { fatalError() }
 
-@abi(func param01_generic11<T>() -> T) // expected-error {{cannot give global function 'param01_generic11' the ABI of a global function with a different number of low-level parameters}}
+@abi(func param01_generic11<T>() -> T) // expected-error {{cannot give global function 'param01_generic11' the ABI of a global function with a different number of parameters}}
 func param01_generic11<T>(_: Int) -> T { fatalError() }
 
 @abi(func param11_generic11<T>(_: Int) -> T)
 func param11_generic11<T>(_: Int) -> T { fatalError() }
 
+
+
+struct SubscriptArity {
+  @abi(subscript(param11_generic00 _: Int) -> Int)
+  subscript(param11_generic00 _: Int) -> Int { 0 }
+
+  @abi(subscript(param21_generic00 _: Int, _: Int) -> Int) // expected-error {{cannot give subscript 'subscript(param21_generic00:)' the ABI of a subscript with a different number of parameters}}
+  subscript(param21_generic00 _: Int) -> Int { 0 }
+
+  @abi(subscript(param12_generic00 _: Int) -> Int) // expected-error {{cannot give subscript 'subscript(param12_generic00:_:)' the ABI of a subscript with a different number of parameters}}
+  subscript(param12_generic00 _: Int, _: Int) -> Int { 0 }
+
+  @abi(subscript(param22_generic00 _: Int, _: Int) -> Int)
+  subscript(param22_generic00 _: Int, _: Int) -> Int { 0 }
+
+  @abi(subscript<T>(param11_generic10 _: T) -> Int) // expected-error {{declaration in '@abi' should not have generic signature because 'subscript(param11_generic10:)' is not generic}}
+  subscript(param11_generic10 _: Int) -> Int { 0 }
+
+  @abi(subscript<T>(param21_generic10 _: T, _: Int) -> Int) // expected-error {{declaration in '@abi' should not have generic signature because 'subscript(param21_generic10:)' is not generic}}
+  subscript(param21_generic10 _: Int) -> Int { 0 }
+
+  @abi(subscript<T>(param12_generic10 _: T) -> Int) // expected-error {{declaration in '@abi' should not have generic signature because 'subscript(param12_generic10:_:)' is not generic}}
+  subscript(param12_generic10 _: Int, _: Int) -> Int { 0 }
+
+  @abi(subscript<T>(param22_generic10 _: T, _: Int) -> Int) // expected-error {{declaration in '@abi' should not have generic signature because 'subscript(param22_generic10:_:)' is not generic}}
+  subscript(param22_generic10 _: Int, _: Int) -> Int { 0 }
+
+  @abi(subscript(param11_generic01 _: Int) -> Int) // expected-error {{declaration in '@abi' should have generic signature compatible with '<T where T : Copyable, T : Escapable>'}}
+  subscript<T>(param11_generic01 _: T) -> Int { 0 }
+
+  @abi(subscript(param21_generic01 _: Int, _: Int) -> Int) // expected-error {{declaration in '@abi' should have generic signature compatible with '<T where T : Copyable, T : Escapable>'}}
+  subscript<T>(param21_generic01 _: T) -> Int { 0 }
+
+  @abi(subscript(param12_generic01 _: Int) -> Int) // expected-error {{declaration in '@abi' should have generic signature compatible with '<T where T : Copyable, T : Escapable>'}}
+  subscript<T>(param12_generic01 _: T, _: Int) -> Int { 0 }
+
+  @abi(subscript(param22_generic01 _: Int, _: Int) -> Int) // expected-error {{declaration in '@abi' should have generic signature compatible with '<T where T : Copyable, T : Escapable>'}}
+  subscript<T>(param22_generic01 _: T, _: Int) -> Int { 0 }
+
+  @abi(subscript<T>(param11_generic11 _: T) -> Int)
+  subscript<T>(param11_generic11 _: T) -> Int { 0 }
+
+  @abi(subscript<T>(param21_generic11 _: T, _: Int) -> Int) // expected-error {{cannot give subscript 'subscript(param21_generic11:)' the ABI of a subscript with a different number of parameters}}
+  subscript<T>(param21_generic11 _: T) -> Int { 0 }
+
+  @abi(subscript<T>(param12_generic11 _: T) -> Int) // expected-error {{cannot give subscript 'subscript(param12_generic11:_:)' the ABI of a subscript with a different number of parameters}}
+  subscript<T>(param12_generic11 _: T, _: Int) -> Int { 0 }
+
+  @abi(subscript<T>(param22_generic11 _: T, _: Int) -> Int)
+  subscript<T>(param22_generic11 _: T, _: Int) -> Int { 0 }
+}
+
 //
 // Throws effect checking
 //
 
+enum MyError: Error {}
+
 @abi(func throws00(_: () throws -> Void))
 func throws00(_: () throws -> Void) {}
 
@@ -90,6 +175,9 @@ func throws10(_: () throws -> Void) {}
 @abi(func throws20(_: () throws -> Void) rethrows) // expected-error {{cannot give 'throws20' the ABI of a global function which can throw}}
 func throws20(_: () throws -> Void) {}
 
+@abi(func throws30(_: () throws -> Void) throws(MyError)) // expected-error {{cannot give 'throws30' the ABI of a global function which can throw}}
+func throws30(_: () throws -> Void) {}
+
 @abi(func throws01(_: () throws -> Void)) // expected-error {{cannot give 'throws01' the ABI of a global function which cannot throw}}
 func throws01(_: () throws -> Void) throws {}
 
@@ -99,6 +187,9 @@ func throws11(_: () throws -> Void) throws {}
 @abi(func throws21(_: () throws -> Void) rethrows)
 func throws21(_: () throws -> Void) throws {}
 
+@abi(func throws31(_: () throws -> Void) throws(MyError)) // expected-error {{thrown type 'MyError' in '@abi' should match 'any Error'}}
+func throws31(_: () throws -> Void) throws {} // expected-note@:37 {{should match type here}}
+
 @abi(func throws02(_: () throws -> Void)) // expected-error {{cannot give 'throws02' the ABI of a global function which cannot throw}}
 func throws02(_: () throws -> Void) rethrows {}
 
@@ -108,6 +199,61 @@ func throws12(_: () throws -> Void) rethrows {}
 @abi(func throws22(_: () throws -> Void) rethrows)
 func throws22(_: () throws -> Void) rethrows {}
 
+@abi(func throws32(_: () throws -> Void) throws(MyError)) // expected-error {{thrown type 'MyError' in '@abi' should match 'any Error'}}
+func throws32(_: () throws -> Void) rethrows {} // expected-note@:37 {{should match type here}}
+
+@abi(func throws03(_: () throws -> Void)) // expected-error {{cannot give 'throws03' the ABI of a global function which cannot throw}}
+func throws03(_: () throws -> Void) throws(MyError) {}
+
+@abi(func throws13(_: () throws -> Void) throws) // expected-error {{thrown type 'any Error' in '@abi' should match 'MyError'}}
+func throws13(_: () throws -> Void) throws(MyError) {} // expected-note@:37 {{should match type here}}
+
+@abi(func throws23(_: () throws -> Void) rethrows) // expected-error {{thrown type 'any Error' in '@abi' should match 'MyError'}}
+func throws23(_: () throws -> Void) throws(MyError) {} // expected-note@:37 {{should match type here}}
+
+@abi(func throws33(_: () throws -> Void) throws(MyError))
+func throws33(_: () throws -> Void) throws(MyError) {}
+
+@abi(var throws00Var: Int)
+var throws00Var: Int { get { fatalError() } }
+
+@abi(var throws11Var: Int)
+var throws11Var: Int { get throws { fatalError() } }
+
+enum ErsatzResult<Success, Failure: Error> {}
+
+extension ErsatzResult where Failure == Swift.Error {
+  // The `where` clause makes `throws(Failure)` equivalent to `throws`.
+
+  // Similar to Swift.Result.init(__untyped_throws_catching:)
+  @abi(
+    init(
+      catching body: () throws -> Success
+    )
+  )
+  init(
+    __untyped_throws_catching body: () throws(Failure) -> Success
+  ) {}
+
+  @abi(func get() throws -> Success)
+  func __untyped_throws_get() throws(Failure) -> Success { fatalError() }
+}
+
+extension ErsatzResult {
+  // Should not be allowed, as `Failure` is still generic
+  @abi(
+    init(
+      unconstrainedCatching body: () throws -> Success // expected-error {{parameter 'body' type '() throws -> Success' in '@abi' should match '() throws(Failure) -> Success'}}
+    )
+  )
+  init(
+    __untyped_throws_catching_bad body: () throws(Failure) -> Success // expected-note {{should match type here}}
+  ) {}
+
+  @abi(func unconstrainedGet() throws -> Success) // expected-error @:32 {{thrown type 'any Error' in '@abi' should match 'Failure'}}
+  func __untyped_throws_get_bad() throws(Failure) -> Success { fatalError() } // expected-note {{should match type here}}
+}
+
 //
 // Async effect checking
 //
@@ -124,13 +270,11 @@ func async01() async {}
 @abi(func async11() async)
 func async11() async {}
 
-//
-// Miscellaneous function checking
-//
+@abi(var async00Var: Int)
+var async00Var: Int { get { fatalError() } }
 
-@_silgen_name("conflictingAttrsSilgenName")
-@abi(func conflictingAttrsABI())
-func conflictingAttrsAPI() {} // expected-error@-2 {{cannot use '@_silgen_name' and '@abi' on the same global function because they serve the same purpose}} {{1-44=}}
+@abi(var async11Var: Int)
+var async11Var: Int { get async { fatalError() } }
 
 //
 // PBD shape checking
@@ -149,7 +293,7 @@ var (x3, y3): (Int, Int) = (0, 0), a3: Int = 0
 var (x4, y4): (Int, Int) = (0, 0), (a4, b4): (Int, Int) = (0, 0) // expected-error {{no match for var 'b4' in the ABI}}
 
 //
-// Conflict diagnostics
+// Redeclaration diagnostics
 //
 
 @abi(func noConflictWithSelf())
@@ -283,6 +427,1870 @@ func fn() {
   var var_conflictsWithNonABI: Int = 0 // expected-error {{invalid redeclaration of 'var_conflictsWithNonABI'}}
 }
 
+//
+// Type differences
+//
+
+@abi(func floatForIntParam(_ a: Float) -> Int) // expected-error @:33 {{parameter 'a' type 'Float' in '@abi' should match 'Int'}}
+func intForFloatParam(_: Int) -> Int { fatalError() } // expected-note @:26 {{should match type here}}
+
+@abi(func floatForIntResult(_ a: Int) -> Float) // expected-error @:42 {{result type 'Float' in '@abi' should match 'Int'}}
+func intForFloatResult(_: Int) -> Int { fatalError() } // expected-note @:35 {{should match type here}}
+
+@abi(func labeledForUnlabeledTuple(_: (x: Int, y: Int)))
+func labeledForUnlabeledTuple(_: (Int, Int)) {}
+
+@abi(func unlabeledForLabeledTuple(_: (Int, Int)))
+func unlabeledForLabeledTuple(_: (x: Int, y: Int)) {}
+
+@abi(func labeledForLabeledTuple(_: (x: Int, y: Int)))
+func labeledForLabeledTuple(_: (a: Int, b: Int)) {}
+
+@abi(
+  func testDefaultArguments(
+    a: Int,
+    b: Int = 1, // expected-error {{'b' in '@abi' should not have a default argument; it does not affect the parameter's ABI}}
+    c: Int,
+    d: Int = 2 // expected-error {{'d' in '@abi' should not have a default argument; it does not affect the parameter's ABI}}
+  )
+)
+func testDefaultArguments(
+  a: Int,
+  b: Int,
+  c: Int = 1,
+  d: Int = 2
+) {}
+
+@abi(func arrayForVariadicParam(a: [Int], b: Set<Float>)) // expected-error @:46 {{parameter 'b' type 'Set<Float>' in '@abi' should match 'Float...'}}
+func arrayForVariadicParam(a: Int..., b: Float...) {} // expected-note @:42 {{should match type here}}
+
+struct DefaultParamOwnership {
+  @abi(
+    func method(
+      _ a: AnyObject,
+      _ b: inout AnyObject, // expected-error {{modifier 'inout' on parameter 'b' in '@abi' is not compatible with default}}
+      _ c: borrowing AnyObject,
+      _ d: consuming AnyObject, // expected-error {{modifier 'consuming' on parameter 'd' in '@abi' is not compatible with default}}
+      _ e: __shared AnyObject,
+      _ f: __owned AnyObject, // expected-error {{modifier '__owned' on parameter 'f' in '@abi' is not compatible with default}}
+      _ g: sending AnyObject, // expected-error {{modifier 'sending' on parameter 'g' in '@abi' is not compatible with default}}
+      _ h: (AnyObject) -> Void,
+      _ i: (borrowing AnyObject) -> Void,
+      _ j: (consuming AnyObject) -> Void // expected-error {{parameter 'j' type '(consuming AnyObject) -> Void' in '@abi' should match '(AnyObject) -> Void'}}
+    )
+  )
+  func method(
+    _: AnyObject,
+    _: AnyObject, // expected-note {{should match type here}}
+    _: AnyObject,
+    _: AnyObject, // expected-note {{should match type here}}
+    _: AnyObject,
+    _: AnyObject, // expected-note {{should match type here}}
+    _: AnyObject, // expected-note {{should match type here}}
+    _: (AnyObject) -> Void,
+    _: (AnyObject) -> Void,
+    _: (AnyObject) -> Void // expected-note {{should match type here}}
+  ) {}
+
+  @abi(
+    init(
+      _ a: AnyObject,
+      _ b: inout AnyObject, // expected-error {{modifier 'inout' on parameter 'b' in '@abi' is not compatible with default}}
+      _ c: borrowing AnyObject, // expected-error {{modifier 'borrowing' on parameter 'c' in '@abi' is not compatible with default}}
+      _ d: consuming AnyObject,
+      _ e: __shared AnyObject, // expected-error {{modifier '__shared' on parameter 'e' in '@abi' is not compatible with default}}
+      _ f: __owned AnyObject,
+      _ g: sending AnyObject,
+      _ h: (AnyObject) -> Void,
+      _ i: (borrowing AnyObject) -> Void,
+      _ j: (consuming AnyObject) -> Void // expected-error {{parameter 'j' type '(consuming AnyObject) -> Void' in '@abi' should match '(AnyObject) -> Void'}}
+    )
+  )
+  init(
+    _: AnyObject,
+    _: AnyObject, // expected-note {{should match type here}}
+    _: AnyObject, // expected-note {{should match type here}}
+    _: AnyObject,
+    _: AnyObject, // expected-note {{should match type here}}
+    _: AnyObject,
+    _: AnyObject,
+    _: (AnyObject) -> Void,
+    _: (AnyObject) -> Void,
+    _: (AnyObject) -> Void // expected-note {{should match type here}}
+  ) {}
+}
+
+struct InoutParamOwnership {
+  @abi(
+    func method(
+      _ a: AnyObject, // expected-error {{default modifier on parameter 'a' in '@abi' is not compatible with 'inout'}}
+      _ b: inout AnyObject,
+      _ c: borrowing AnyObject, // expected-error {{modifier 'borrowing' on parameter 'c' in '@abi' is not compatible with 'inout'}}
+      _ d: consuming AnyObject, // expected-error {{modifier 'consuming' on parameter 'd' in '@abi' is not compatible with 'inout'}}
+      _ e: __shared AnyObject, // expected-error {{modifier '__shared' on parameter 'e' in '@abi' is not compatible with 'inout'}}
+      _ f: __owned AnyObject, // expected-error {{modifier '__owned' on parameter 'f' in '@abi' is not compatible with 'inout'}}
+      _ g: sending AnyObject, // expected-error {{modifier 'sending' on parameter 'g' in '@abi' is not compatible with 'inout'}}
+      _ h: (AnyObject) -> Void, // expected-error {{parameter 'h' type '(AnyObject) -> Void' in '@abi' should match '(inout AnyObject) -> Void'}}
+      _ i: (borrowing AnyObject) -> Void, // expected-error {{parameter 'i' type '(borrowing AnyObject) -> Void' in '@abi' should match '(inout AnyObject) -> Void'}}
+      _ j: (consuming AnyObject) -> Void // expected-error {{parameter 'j' type '(consuming AnyObject) -> Void' in '@abi' should match '(inout AnyObject) -> Void'}}
+    )
+  )
+  func method(
+    _: inout AnyObject, // expected-note {{should match type here}}
+    _: inout AnyObject,
+    _: inout AnyObject, // expected-note {{should match type here}}
+    _: inout AnyObject, // expected-note {{should match type here}}
+    _: inout AnyObject, // expected-note {{should match type here}}
+    _: inout AnyObject, // expected-note {{should match type here}}
+    _: inout AnyObject, // expected-note {{should match type here}}
+    _: (inout AnyObject) -> Void, // expected-note {{should match type here}}
+    _: (inout AnyObject) -> Void, // expected-note {{should match type here}}
+    _: (inout AnyObject) -> Void // expected-note {{should match type here}}
+  ) {}
+
+  @abi(
+    init(
+      _ a: AnyObject, // expected-error {{default modifier on parameter 'a' in '@abi' is not compatible with 'inout'}}
+      _ b: inout AnyObject,
+      _ c: borrowing AnyObject, // expected-error {{modifier 'borrowing' on parameter 'c' in '@abi' is not compatible with 'inout'}}
+      _ d: consuming AnyObject, // expected-error {{modifier 'consuming' on parameter 'd' in '@abi' is not compatible with 'inout'}}
+      _ e: __shared AnyObject, // expected-error {{modifier '__shared' on parameter 'e' in '@abi' is not compatible with 'inout'}}
+      _ f: __owned AnyObject, // expected-error {{modifier '__owned' on parameter 'f' in '@abi' is not compatible with 'inout'}}
+      _ g: sending AnyObject, // expected-error {{modifier 'sending' on parameter 'g' in '@abi' is not compatible with 'inout'}}
+      _ h: (AnyObject) -> Void, // expected-error {{parameter 'h' type '(AnyObject) -> Void' in '@abi' should match '(inout AnyObject) -> Void'}}
+      _ i: (borrowing AnyObject) -> Void, // expected-error {{parameter 'i' type '(borrowing AnyObject) -> Void' in '@abi' should match '(inout AnyObject) -> Void'}}
+      _ j: (consuming AnyObject) -> Void // expected-error {{parameter 'j' type '(consuming AnyObject) -> Void' in '@abi' should match '(inout AnyObject) -> Void'}}
+    )
+  )
+  init(
+    _: inout AnyObject, // expected-note {{should match type here}}
+    _: inout AnyObject,
+    _: inout AnyObject, // expected-note {{should match type here}}
+    _: inout AnyObject, // expected-note {{should match type here}}
+    _: inout AnyObject, // expected-note {{should match type here}}
+    _: inout AnyObject, // expected-note {{should match type here}}
+    _: inout AnyObject, // expected-note {{should match type here}}
+    _: (inout AnyObject) -> Void, // expected-note {{should match type here}}
+    _: (inout AnyObject) -> Void, // expected-note {{should match type here}}
+    _: (inout AnyObject) -> Void // expected-note {{should match type here}}
+  ) {}
+}
+
+struct BorrowingParamOwnership {
+  @abi(
+    func method(
+      _ a: AnyObject,
+      _ b: inout AnyObject, // expected-error {{modifier 'inout' on parameter 'b' in '@abi' is not compatible with 'borrowing'}}
+      _ c: borrowing AnyObject,
+      _ d: consuming AnyObject, // expected-error {{modifier 'consuming' on parameter 'd' in '@abi' is not compatible with 'borrowing'}}
+      _ e: __shared AnyObject,
+      _ f: __owned AnyObject, // expected-error {{modifier '__owned' on parameter 'f' in '@abi' is not compatible with 'borrowing'}}
+      _ g: sending AnyObject, // expected-error {{modifier 'sending' on parameter 'g' in '@abi' is not compatible with 'borrowing'}}
+      _ h: (AnyObject) -> Void,
+      _ i: (borrowing AnyObject) -> Void,
+      _ j: (consuming AnyObject) -> Void // expected-error {{parameter 'j' type '(consuming AnyObject) -> Void' in '@abi' should match '(borrowing AnyObject) -> Void'}}
+    )
+  )
+  func method(
+    _: borrowing AnyObject,
+    _: borrowing AnyObject, // expected-note {{should match type here}}
+    _: borrowing AnyObject,
+    _: borrowing AnyObject, // expected-note {{should match type here}}
+    _: borrowing AnyObject,
+    _: borrowing AnyObject, // expected-note {{should match type here}}
+    _: borrowing AnyObject, // expected-note {{should match type here}}
+    _: (borrowing AnyObject) -> Void,
+    _: (borrowing AnyObject) -> Void,
+    _: (borrowing AnyObject) -> Void // expected-note {{should match type here}}
+  ) {}
+
+  @abi(
+    init(
+      _ a: AnyObject, // expected-error {{default modifier on parameter 'a' in '@abi' is not compatible with 'borrowing'}}
+      _ b: inout AnyObject, // expected-error {{modifier 'inout' on parameter 'b' in '@abi' is not compatible with 'borrowing'}}
+      _ c: borrowing AnyObject,
+      _ d: consuming AnyObject, // expected-error {{modifier 'consuming' on parameter 'd' in '@abi' is not compatible with 'borrowing'}}
+      _ e: __shared AnyObject,
+      _ f: __owned AnyObject, // expected-error {{modifier '__owned' on parameter 'f' in '@abi' is not compatible with 'borrowing'}}
+      _ g: sending AnyObject, // expected-error {{modifier 'sending' on parameter 'g' in '@abi' is not compatible with 'borrowing'}}
+      _ h: (AnyObject) -> Void,
+      _ i: (borrowing AnyObject) -> Void,
+      _ j: (consuming AnyObject) -> Void // expected-error {{parameter 'j' type '(consuming AnyObject) -> Void' in '@abi' should match '(borrowing AnyObject) -> Void'}}
+    )
+  )
+  init(
+    _: borrowing AnyObject, // expected-note {{should match type here}}
+    _: borrowing AnyObject, // expected-note {{should match type here}}
+    _: borrowing AnyObject,
+    _: borrowing AnyObject, // expected-note {{should match type here}}
+    _: borrowing AnyObject,
+    _: borrowing AnyObject, // expected-note {{should match type here}}
+    _: borrowing AnyObject, // expected-note {{should match type here}}
+    _: (borrowing AnyObject) -> Void,
+    _: (borrowing AnyObject) -> Void,
+    _: (borrowing AnyObject) -> Void // expected-note {{should match type here}}
+  ) {}
+}
+
+struct ConsumingParamOwnership {
+  @abi(
+    func method(
+      _ a: AnyObject, // expected-error {{default modifier on parameter 'a' in '@abi' is not compatible with 'consuming'}}
+      _ b: inout AnyObject, // expected-error {{modifier 'inout' on parameter 'b' in '@abi' is not compatible with 'consuming'}}
+      _ c: borrowing AnyObject, // expected-error {{modifier 'borrowing' on parameter 'c' in '@abi' is not compatible with 'consuming'}}
+      _ d: consuming AnyObject,
+      _ e: __shared AnyObject, // expected-error {{modifier '__shared' on parameter 'e' in '@abi' is not compatible with 'consuming'}}
+      _ f: __owned AnyObject,
+      _ g: sending AnyObject,
+      _ h: (AnyObject) -> Void, // expected-error {{parameter 'h' type '(AnyObject) -> Void' in '@abi' should match '(consuming AnyObject) -> Void'}}
+      _ i: (borrowing AnyObject) -> Void, // expected-error {{parameter 'i' type '(borrowing AnyObject) -> Void' in '@abi' should match '(consuming AnyObject) -> Void'}}
+      _ j: (consuming AnyObject) -> Void
+    )
+  )
+  func method(
+    _: consuming AnyObject, // expected-note {{should match type here}}
+    _: consuming AnyObject, // expected-note {{should match type here}}
+    _: consuming AnyObject, // expected-note {{should match type here}}
+    _: consuming AnyObject,
+    _: consuming AnyObject, // expected-note {{should match type here}}
+    _: consuming AnyObject,
+    _: consuming AnyObject,
+    _: (consuming AnyObject) -> Void, // expected-note {{should match type here}}
+    _: (consuming AnyObject) -> Void, // expected-note {{should match type here}}
+    _: (consuming AnyObject) -> Void
+  ) {}
+
+  @abi(
+    init(
+      _ a: AnyObject,
+      _ b: inout AnyObject, // expected-error {{modifier 'inout' on parameter 'b' in '@abi' is not compatible with 'consuming'}}
+      _ c: borrowing AnyObject, // expected-error {{modifier 'borrowing' on parameter 'c' in '@abi' is not compatible with 'consuming'}}
+      _ d: consuming AnyObject,
+      _ e: __shared AnyObject, // expected-error {{modifier '__shared' on parameter 'e' in '@abi' is not compatible with 'consuming'}}
+      _ f: __owned AnyObject,
+      _ g: sending AnyObject,
+      _ h: (AnyObject) -> Void, // expected-error {{parameter 'h' type '(AnyObject) -> Void' in '@abi' should match '(consuming AnyObject) -> Void'}}
+      _ i: (borrowing AnyObject) -> Void, // expected-error {{parameter 'i' type '(borrowing AnyObject) -> Void' in '@abi' should match '(consuming AnyObject) -> Void'}}
+      _ j: (consuming AnyObject) -> Void
+    )
+  )
+  init(
+    _: consuming AnyObject,
+    _: consuming AnyObject, // expected-note {{should match type here}}
+    _: consuming AnyObject, // expected-note {{should match type here}}
+    _: consuming AnyObject,
+    _: consuming AnyObject, // expected-note {{should match type here}}
+    _: consuming AnyObject,
+    _: consuming AnyObject,
+    _: (consuming AnyObject) -> Void, // expected-note {{should match type here}}
+    _: (consuming AnyObject) -> Void, // expected-note {{should match type here}}
+    _: (consuming AnyObject) -> Void
+  ) {}
+}
+
+struct SharedParamOwnership {
+  @abi(
+    func method(
+      _ a: AnyObject,
+      _ b: inout AnyObject, // expected-error {{modifier 'inout' on parameter 'b' in '@abi' is not compatible with '__shared'}}
+      _ c: borrowing AnyObject,
+      _ d: consuming AnyObject, // expected-error {{modifier 'consuming' on parameter 'd' in '@abi' is not compatible with '__shared'}}
+      _ e: __shared AnyObject,
+      _ f: __owned AnyObject, // expected-error {{modifier '__owned' on parameter 'f' in '@abi' is not compatible with '__shared'}}
+      _ g: sending AnyObject, // expected-error {{modifier 'sending' on parameter 'g' in '@abi' is not compatible with '__shared'}}
+      _ h: (AnyObject) -> Void,
+      _ i: (borrowing AnyObject) -> Void,
+      _ j: (consuming AnyObject) -> Void // expected-error {{parameter 'j' type '(consuming AnyObject) -> Void' in '@abi' should match '(__shared AnyObject) -> Void'}}
+    )
+  )
+  func method(
+    _: __shared AnyObject,
+    _: __shared AnyObject, // expected-note {{should match type here}}
+    _: __shared AnyObject,
+    _: __shared AnyObject, // expected-note {{should match type here}}
+    _: __shared AnyObject,
+    _: __shared AnyObject, // expected-note {{should match type here}}
+    _: __shared AnyObject, // expected-note {{should match type here}}
+    _: (__shared AnyObject) -> Void,
+    _: (__shared AnyObject) -> Void,
+    _: (__shared AnyObject) -> Void // expected-note {{should match type here}}
+  ) {}
+
+  @abi(
+    init(
+      _ a: AnyObject, // expected-error {{default modifier on parameter 'a' in '@abi' is not compatible with '__shared'}}
+      _ b: inout AnyObject, // expected-error {{modifier 'inout' on parameter 'b' in '@abi' is not compatible with '__shared'}}
+      _ c: borrowing AnyObject,
+      _ d: consuming AnyObject, // expected-error {{modifier 'consuming' on parameter 'd' in '@abi' is not compatible with '__shared'}}
+      _ e: __shared AnyObject,
+      _ f: __owned AnyObject, // expected-error {{modifier '__owned' on parameter 'f' in '@abi' is not compatible with '__shared'}}
+      _ g: sending AnyObject, // expected-error {{modifier 'sending' on parameter 'g' in '@abi' is not compatible with '__shared'}}
+      _ h: (AnyObject) -> Void,
+      _ i: (borrowing AnyObject) -> Void,
+      _ j: (consuming AnyObject) -> Void // expected-error {{parameter 'j' type '(consuming AnyObject) -> Void' in '@abi' should match '(__shared AnyObject) -> Void'}}
+    )
+  )
+  init(
+    _: __shared AnyObject, // expected-note {{should match type here}}
+    _: __shared AnyObject, // expected-note {{should match type here}}
+    _: __shared AnyObject,
+    _: __shared AnyObject, // expected-note {{should match type here}}
+    _: __shared AnyObject,
+    _: __shared AnyObject, // expected-note {{should match type here}}
+    _: __shared AnyObject, // expected-note {{should match type here}}
+    _: (__shared AnyObject) -> Void,
+    _: (__shared AnyObject) -> Void,
+    _: (__shared AnyObject) -> Void // expected-note {{should match type here}}
+  ) {}
+}
+
+struct OwnedParamOwnership {
+  @abi(
+    func method(
+      _ a: AnyObject, // expected-error {{default modifier on parameter 'a' in '@abi' is not compatible with '__owned'}}
+      _ b: inout AnyObject, // expected-error {{modifier 'inout' on parameter 'b' in '@abi' is not compatible with '__owned'}}
+      _ c: borrowing AnyObject, // expected-error {{modifier 'borrowing' on parameter 'c' in '@abi' is not compatible with '__owned'}}
+      _ d: consuming AnyObject,
+      _ e: __shared AnyObject, // expected-error {{modifier '__shared' on parameter 'e' in '@abi' is not compatible with '__owned'}}
+      _ f: __owned AnyObject,
+      _ g: sending AnyObject,
+      _ h: (AnyObject) -> Void, // expected-error {{parameter 'h' type '(AnyObject) -> Void' in '@abi' should match '(__owned AnyObject) -> Void'}}
+      _ i: (borrowing AnyObject) -> Void, // expected-error {{parameter 'i' type '(borrowing AnyObject) -> Void' in '@abi' should match '(__owned AnyObject) -> Void'}}
+      _ j: (consuming AnyObject) -> Void
+    )
+  )
+  func method(
+    _: __owned AnyObject, // expected-note {{should match type here}}
+    _: __owned AnyObject, // expected-note {{should match type here}}
+    _: __owned AnyObject, // expected-note {{should match type here}}
+    _: __owned AnyObject,
+    _: __owned AnyObject, // expected-note {{should match type here}}
+    _: __owned AnyObject,
+    _: __owned AnyObject,
+    _: (__owned AnyObject) -> Void, // expected-note {{should match type here}}
+    _: (__owned AnyObject) -> Void, // expected-note {{should match type here}}
+    _: (__owned AnyObject) -> Void
+  ) {}
+
+  @abi(
+    init(
+      _ a: AnyObject,
+      _ b: inout AnyObject, // expected-error {{modifier 'inout' on parameter 'b' in '@abi' is not compatible with '__owned'}}
+      _ c: borrowing AnyObject, // expected-error {{modifier 'borrowing' on parameter 'c' in '@abi' is not compatible with '__owned'}}
+      _ d: consuming AnyObject,
+      _ e: __shared AnyObject, // expected-error {{modifier '__shared' on parameter 'e' in '@abi' is not compatible with '__owned'}}
+      _ f: __owned AnyObject,
+      _ g: sending AnyObject,
+      _ h: (AnyObject) -> Void, // expected-error {{parameter 'h' type '(AnyObject) -> Void' in '@abi' should match '(__owned AnyObject) -> Void'}}
+      _ i: (borrowing AnyObject) -> Void, // expected-error {{parameter 'i' type '(borrowing AnyObject) -> Void' in '@abi' should match '(__owned AnyObject) -> Void'}}
+      _ j: (consuming AnyObject) -> Void
+    )
+  )
+  init(
+    _: __owned AnyObject,
+    _: __owned AnyObject, // expected-note {{should match type here}}
+    _: __owned AnyObject, // expected-note {{should match type here}}
+    _: __owned AnyObject,
+    _: __owned AnyObject, // expected-note {{should match type here}}
+    _: __owned AnyObject,
+    _: __owned AnyObject,
+    _: (__owned AnyObject) -> Void, // expected-note {{should match type here}}
+    _: (__owned AnyObject) -> Void, // expected-note {{should match type here}}
+    _: (__owned AnyObject) -> Void
+  ) {}
+}
+
+struct SendingParamOwnership {
+  @abi(
+    func method(
+      _ a: AnyObject, // expected-error {{default modifier on parameter 'a' in '@abi' is not compatible with 'sending'}}
+      _ b: inout AnyObject, // expected-error {{modifier 'inout' on parameter 'b' in '@abi' is not compatible with 'sending'}}
+      _ c: borrowing AnyObject, // expected-error {{modifier 'borrowing' on parameter 'c' in '@abi' is not compatible with 'sending'}}
+      _ d: consuming AnyObject,
+      _ e: __shared AnyObject, // expected-error {{modifier '__shared' on parameter 'e' in '@abi' is not compatible with 'sending'}}
+      _ f: __owned AnyObject,
+      _ g: sending AnyObject,
+      _ h: (AnyObject) -> Void, // expected-error {{parameter 'h' type '(AnyObject) -> Void' in '@abi' should match '(sending AnyObject) -> Void'}}
+      _ i: (borrowing AnyObject) -> Void, // expected-error {{parameter 'i' type '(borrowing AnyObject) -> Void' in '@abi' should match '(sending AnyObject) -> Void'}}
+      _ j: (consuming AnyObject) -> Void
+    )
+  )
+  func method(
+    _: sending AnyObject, // expected-note {{should match type here}}
+    _: sending AnyObject, // expected-note {{should match type here}}
+    _: sending AnyObject, // expected-note {{should match type here}}
+    _: sending AnyObject,
+    _: sending AnyObject, // expected-note {{should match type here}}
+    _: sending AnyObject,
+    _: sending AnyObject,
+    _: (sending AnyObject) -> Void, // expected-note {{should match type here}}
+    _: (sending AnyObject) -> Void, // expected-note {{should match type here}}
+    _: (sending AnyObject) -> Void
+  ) {}
+
+  @abi(
+    init(
+      _ a: AnyObject,
+      _ b: inout AnyObject, // expected-error {{modifier 'inout' on parameter 'b' in '@abi' is not compatible with 'sending'}}
+      _ c: borrowing AnyObject, // expected-error {{modifier 'borrowing' on parameter 'c' in '@abi' is not compatible with 'sending'}}
+      _ d: consuming AnyObject,
+      _ e: __shared AnyObject, // expected-error {{modifier '__shared' on parameter 'e' in '@abi' is not compatible with 'sending'}}
+      _ f: __owned AnyObject,
+      _ g: sending AnyObject,
+      _ h: (AnyObject) -> Void, // expected-error {{parameter 'h' type '(AnyObject) -> Void' in '@abi' should match '(sending AnyObject) -> Void'}}
+      _ i: (borrowing AnyObject) -> Void, // expected-error {{parameter 'i' type '(borrowing AnyObject) -> Void' in '@abi' should match '(sending AnyObject) -> Void'}}
+      _ j: (consuming AnyObject) -> Void
+    )
+  )
+  init(
+    _: sending AnyObject,
+    _: sending AnyObject, // expected-note {{should match type here}}
+    _: sending AnyObject, // expected-note {{should match type here}}
+    _: sending AnyObject,
+    _: sending AnyObject, // expected-note {{should match type here}}
+    _: sending AnyObject,
+    _: sending AnyObject,
+    _: (sending AnyObject) -> Void, // expected-note {{should match type here}}
+    _: (sending AnyObject) -> Void, // expected-note {{should match type here}}
+    _: (sending AnyObject) -> Void
+  ) {}
+}
+
+// @autoclosure should be flattened away
+@abi(
+  func autoclosureTest(
+    _: @autoclosure () -> Void,
+    _: () -> Void,
+    _: @autoclosure () -> Void,
+    _: () -> Void
+  )
+)
+func autoclosureTest(
+  _: @autoclosure () -> Void,
+  _: @autoclosure () -> Void,
+  _: () -> Void,
+  _: () -> Void
+) {}
+
+// @_nonEphemeral should be flattened away
+// (the diagnostic we get on these is actually part of attr checking)
+@abi(
+  func nonEphemeralTest(
+    @_nonEphemeral _: UnsafeRawPointer, // expected-error {{unused '_nonEphemeral' attribute in '@abi'}}
+    _: UnsafeRawPointer,
+    @_nonEphemeral _: UnsafeRawPointer, // expected-error {{unused '_nonEphemeral' attribute in '@abi'}}
+    _: UnsafeRawPointer
+  )
+)
+func nonEphemeralTest(
+  @_nonEphemeral _: UnsafeRawPointer,
+  @_nonEphemeral _: UnsafeRawPointer,
+  _: UnsafeRawPointer,
+  _: UnsafeRawPointer
+) {}
+
+// isolated param should be flattened away
+@abi(func isolatedTestMismatch<A: Actor>(_: isolated A, _: A))
+func isolatedTestMismatch<A: Actor>(_: A, _: isolated A) {}
+
+@abi(func isolatedTestMatch<A: Actor>(_: isolated A, _: A))
+func isolatedTestMatch<A: Actor>(_: isolated A, _: A) {}
+
+// _const should be flattened away
+@abi(
+  func constTest(
+    _: _const () -> Void,
+    _: () -> Void,
+    _: _const () -> Void,
+    _: () -> Void
+  )
+)
+func constTest(
+  _: _const () -> Void,
+  _: _const () -> Void,
+  _: () -> Void,
+  _: () -> Void
+) {}
+
+// @noDerivative should match
+@abi(func noDerivativeTest1(_ a: @differentiable(reverse) (@noDerivative Double, Double) -> Double))
+func noDerivativeTest1(_: @differentiable(reverse) (@noDerivative Double, Double) -> Double) {}
+
+@abi(func noDerivativeTest2(_ a: @differentiable(reverse) (Double, Double) -> Double)) // expected-error {{parameter 'a' type '@differentiable(reverse) (Double, Double) -> Double' in '@abi' should match '@differentiable(reverse) (@noDerivative Double, Double) -> Double'}}
+func noDerivativeTest2(_: @differentiable(reverse) (@noDerivative Double, Double) -> Double) {} // expected-note {{should match type here}}
+
+@abi(func noDerivativeTest3(_ a: @differentiable(reverse) (@noDerivative Double, Double) -> Double)) // expected-error {{parameter 'a' type '@differentiable(reverse) (@noDerivative Double, Double) -> Double' in '@abi' should match '@differentiable(reverse) (Double, Double) -> Double'}}
+func noDerivativeTest3(_: @differentiable(reverse) (Double, Double) -> Double) {} // expected-note {{should match type here}}
+
+// @_addressable should match
+@abi(
+  func addressableTest(
+    _ a: @_addressable String,
+    _ b: String, // expected-error {{default attribute on parameter 'b' in '@abi' is not compatible with '_addressable'}}
+    _ c: @_addressable String, // expected-error {{attribute '_addressable' on parameter 'c' in '@abi' is not compatible with default}}
+    _ d: String
+  )
+)
+func addressableTest(
+  _: @_addressable String,
+  _: @_addressable String, // expected-note {{should match type here}}
+  _: String, // expected-note {{should match type here}}
+  _: String
+) {}
+
+// Flattening of function type ExtInfo
+@abi(
+  func fnExtInfoTest(
+    _ a: @escaping () -> AnyObject,
+    _ b: @Sendable () -> AnyObject,
+    _ c: () -> sending AnyObject,
+    _ d: () -> AnyObject,
+    _ e: @MainActor () -> AnyObject,
+    _ f: (isolated MainActor) -> AnyObject,
+    _ g: @isolated(any) () -> AnyObject, // expected-error {{parameter 'g' type '@isolated(any) () -> AnyObject' in '@abi' should match '() -> AnyObject'}}
+    _ h: @execution(caller) () async -> AnyObject,
+    _ i: () -> AnyObject, // expected-error {{parameter 'i' type '() -> AnyObject' in '@abi' should match '@isolated(any) () -> AnyObject'}}
+    _ j: () async -> Void,
+    _ k: () -> Void, // expected-error {{parameter 'k' type '() -> Void' in '@abi' should match '() async -> Void'}}
+    _ l: () async -> Void, // expected-error {{parameter 'l' type '() async -> Void' in '@abi' should match '() -> Void'}}
+    _ m: () -> Void,
+    _ n: () throws -> Void,
+    _ o: () -> Void, // expected-error {{parameter 'o' type '() -> Void' in '@abi' should match '() throws -> Void'}}
+    _ p: () throws -> Void, // expected-error {{parameter 'p' type '() throws -> Void' in '@abi' should match '() -> Void'}}
+    _ q: () -> Void,
+    _ r: () -> Void,
+    _ s: @convention(block) () -> Void, // expected-error {{parameter 's' type '@convention(block) () -> Void' in '@abi' should match '() -> Void'}}
+    _ t: @convention(thin) () -> Void, // expected-error {{parameter 't' type '@convention(thin) () -> Void' in '@abi' should match '() -> Void'}}
+    _ u: @convention(c) () -> Void // expected-error {{parameter 'u' type '@convention(c) () -> Void' in '@abi' should match '() -> Void'}}
+  )
+)
+func fnExtInfoTest(
+  _: () -> AnyObject,
+  _: () -> AnyObject,
+  _: () -> AnyObject,
+  _: () -> AnyObject,
+  _: () -> AnyObject,
+  _: (MainActor) -> AnyObject,
+  _: () -> AnyObject, // expected-note {{should match type here}}
+  _: () async -> AnyObject,
+  _: @isolated(any) () -> AnyObject, // expected-note {{should match type here}}
+  _: () async -> Void,
+  _: () async -> Void, // expected-note {{should match type here}}
+  _: () -> Void, // expected-note {{should match type here}}
+  _: () -> Void,
+  _: () throws -> Void,
+  _: () throws -> Void, // expected-note {{should match type here}}
+  _: () -> Void, // expected-note {{should match type here}}
+  _: () -> Void,
+  _: () -> Void,
+  _: () -> Void, // expected-note {{should match type here}}
+  _: () -> Void, // expected-note {{should match type here}}
+  _: () -> Void // expected-note {{should match type here}}
+) {}
+
+// FIXME: Not sure how to reach tryNormalizeOutermostType() generic func
+
+@abi(
+  func testMarkerProtocols<A, B: Sendable>(
+    _: A, _: B,
+    _: Any, _: Sendable,
+    _: AnyKeyPath, _: AnyKeyPath & Sendable,
+    _: Any, _: Sendable & BitwiseCopyable
+  )
+)
+func testMarkerProtocols<A: Sendable, B>(
+  _: A, _: B,
+  _: Sendable, _: Any,
+  _: AnyKeyPath & Sendable, _: AnyKeyPath,
+  _: Sendable & BitwiseCopyable, _: Any
+) {}
+
+@abi(
+  func testNormalProtocols(
+    _ a: Any, // expected-error {{parameter 'a' type 'Any' in '@abi' should match 'any CustomStringConvertible'}}
+    _ b: CustomStringConvertible, // expected-error {{parameter 'b' type 'any CustomStringConvertible' in '@abi' should match 'Any'}}
+    _ c: AnyKeyPath, // expected-error {{parameter 'c' type 'AnyKeyPath' in '@abi' should match 'any AnyKeyPath & CustomStringConvertible'}}
+    _ d: AnyKeyPath & CustomStringConvertible, // expected-error {{parameter 'd' type 'any AnyKeyPath & CustomStringConvertible' in '@abi' should match 'AnyKeyPath'}}
+    _ e: Any, // expected-error {{parameter 'e' type 'Any' in '@abi' should match 'any CustomDebugStringConvertible & CustomStringConvertible'}}
+    _ f: CustomStringConvertible & CustomDebugStringConvertible // expected-error {{parameter 'f' type 'any CustomDebugStringConvertible & CustomStringConvertible' in '@abi' should match 'Any'}}
+  )
+)
+func testNormalProtocols(
+  _: CustomStringConvertible, // expected-note {{should match type here}}
+  _: Any, // expected-note {{should match type here}}
+  _: AnyKeyPath & CustomStringConvertible, // expected-note {{should match type here}}
+  _: AnyKeyPath, // expected-note {{should match type here}}
+  _: CustomStringConvertible & CustomDebugStringConvertible, // expected-note {{should match type here}}
+  _: Any // expected-note {{should match type here}}
+) {}
+
+@abi(
+  func testNormalProtocolsGeneric<A, B: CustomStringConvertible>( // expected-error {{generic signature '<A, B where A : Copyable, A : Escapable, B : CustomStringConvertible>' in '@abi' is not compatible with '<A, B where A : CustomStringConvertible, B : Copyable, B : Escapable>'}}
+    _: A, _: B
+  )
+)
+func testNormalProtocolsGeneric<A: CustomStringConvertible, B>( // expected-note {{should match type here}}
+  _: A, _: B
+) {}
+
+//
+// Static/Instance and interactions with `final`
+//
+
+class ClassStaticAndFinal {
+  @abi(func class00final00())
+  func class00final00() {}
+
+  @abi(class func class10final00()) // expected-error {{class method 'class10final00()' in '@abi' should be instance method to ensure ABI compatibility}} {{8-14=}}
+  func class10final00() {}
+
+  @abi(static func class20final00()) // expected-error {{static method 'class20final00()' in '@abi' should be non-final instance method to ensure ABI compatibility}} {{8-15=}}
+  func class20final00() {}
+
+  @abi(func class01final00()) // expected-error {{instance method 'class01final00()' in '@abi' should be class method to ensure ABI compatibility}} {{8-8=class }}
+  class func class01final00() {}
+
+  @abi(func class02final00()) // expected-error {{non-final instance method 'class02final00()' in '@abi' should be static method to ensure ABI compatibility}} {{8-8=static }}
+  static func class02final00() {}
+
+  @abi(class func class11final00())
+  class func class11final00() {}
+
+  @abi(class func class12final00()) // expected-error {{non-final class method 'class12final00()' in '@abi' should be static method to ensure ABI compatibility}} {{8-13=static}}
+  static func class12final00() {}
+
+  @abi(static func class21final00()) // expected-error {{static method 'class21final00()' in '@abi' should be non-final class method to ensure ABI compatibility}} {{8-14=class}}
+  class func class21final00() {}
+
+  @abi(static func class22final00())
+  static func class22final00() {}
+
+  @abi(final func class00final10()) // expected-error {{final instance method 'class00final10()' in '@abi' should be non-final instance method to ensure ABI compatibility}} {{8-14=}}
+  func class00final10() {}
+
+  @abi(class final func class10final10()) // expected-error {{final class method 'class10final10()' in '@abi' should be non-final instance method to ensure ABI compatibility}} {{8-14=}}
+  func class10final10() {}
+
+  @abi(final func class01final10()) // expected-error {{final instance method 'class01final10()' in '@abi' should be non-final class method to ensure ABI compatibility}} {{8-13=class}}
+  class func class01final10() {}
+
+  @abi(final func class02final10()) // expected-error {{final instance method 'class02final10()' in '@abi' should be static method to ensure ABI compatibility}} {{8-13=static}}
+  static func class02final10() {}
+
+  @abi(class final func class11final10()) // expected-error {{final class method 'class11final10()' in '@abi' should be non-final class method to ensure ABI compatibility}} {{14-20=}}
+  class func class11final10() {}
+
+  @abi(class final func class12final10())
+  static func class12final10() {}
+
+  @abi(func class00final01()) // expected-error {{non-final instance method 'class00final01()' in '@abi' should be final instance method to ensure ABI compatibility}} {{8-8=final }}
+  final func class00final01() {}
+
+  @abi(class func class10final01()) // expected-error {{non-final class method 'class10final01()' in '@abi' should be final instance method to ensure ABI compatibility}} {{8-13=final}}
+  final func class10final01() {}
+
+  @abi(static func class20final01()) // expected-error {{static method 'class20final01()' in '@abi' should be final instance method to ensure ABI compatibility}} {{8-14=final}}
+  final func class20final01() {}
+
+  @abi(func class01final01()) // expected-error {{non-final instance method 'class01final01()' in '@abi' should be final class method to ensure ABI compatibility}} {{8-8=final class }}
+  class final func class01final01() {}
+
+  @abi(class func class11final01()) // expected-error {{non-final class method 'class11final01()' in '@abi' should be final class method to ensure ABI compatibility}} {{8-13=final class}}
+  class final func class11final01() {}
+
+  @abi(static func class21final01())
+  class final func class21final01() {}
+
+  @abi(final func class00final11())
+  final func class00final11() {}
+
+  @abi(class final func class10final11()) //expected-error {{final class method 'class10final11()' in '@abi' should be final instance method to ensure ABI compatibility}} {{14-20=}} {{8-13=final}}
+  final func class10final11() {}
+
+  @abi(final func class01final11()) // expected-error {{final instance method 'class01final11()' in '@abi' should be final class method to ensure ABI compatibility}} {{8-13=final class}}
+  class final func class01final11() {}
+
+  @abi(class final func class11final11())
+  class final func class11final11() {}
+}
+
+struct StaticAndFinal {
+  @abi(func class00final00())
+  func class00final00() {}
+
+  @abi(static func class20final00()) // expected-error {{static method 'class20final00()' in '@abi' should be instance method to ensure ABI compatibility}} {{8-15=}}
+  func class20final00() {}
+
+  @abi(func class02final00()) // expected-error {{instance method 'class02final00()' in '@abi' should be static method to ensure ABI compatibility}} {{8-8=static }}
+  static func class02final00() {}
+
+  @abi(static func class22final00())
+  static func class22final00() {}
+}
+
+//
+// Failable Initializers
+//
+
+struct FailableInits {
+  @abi(init(i11: Void))
+  init(i11: Void) {}
+
+  @abi(init?(i21: Void)) // expected-error {{cannot give non-failable initializer 'init(i21:)' the ABI of a failable initializer}} {{12-13=}}
+  init(i21: Void) {}
+
+  @abi(init!(i31: Void)) // expected-error {{cannot give non-failable initializer 'init(i31:)' the ABI of a failable initializer}} {{12-13=}}
+  init(i31: Void) {}
+
+  @abi(init(i12: Void)) // expected-error {{cannot give failable initializer 'init(i12:)' the ABI of a non-failable initializer}} {{12-12=?}}
+  init?(i12: Void) {}
+
+  @abi(init?(i22: Void))
+  init?(i22: Void) {}
+
+  @abi(init!(i32: Void))
+  init?(i32: Void) {}
+
+  @abi(init(i13: Void)) // expected-error {{cannot give failable initializer 'init(i13:)' the ABI of a non-failable initializer}} {{12-12=!}}
+  init!(i13: Void) {}
+
+  @abi(init?(i23: Void))
+  init!(i23: Void) {}
+
+  @abi(init!(i33: Void))
+  init!(i33: Void) {}
+}
+
+//
+// Attributes
+//
+
+// @_originallyDefinedIn -- allowed to vary
+@abi(@_originallyDefinedIn(module: "Other", macOS 14) func originallyDefinedIn1())
+@available(macOS 12, *) @_originallyDefinedIn(module: "Other", macOS 14) public func originallyDefinedIn1() {}
+
+@abi(func originallyDefinedIn2())
+@available(macOS 12, *) @_originallyDefinedIn(module: "Other", macOS 14) public func originallyDefinedIn2() {}
+
+@abi(@_originallyDefinedIn(module: "Other", macOS 14) func originallyDefinedIn3())
+@available(macOS 12, *) public func originallyDefinedIn3() {}
+
+@abi(@_originallyDefinedIn(module: "Different", macOS 12) func originallyDefinedIn4())
+@available(macOS 12, *) @_originallyDefinedIn(module: "Other", macOS 14) public func originallyDefinedIn4() {}
+
+// @Sendable -- allowed to vary
+@abi(@Sendable func sendable1())
+@Sendable func sendable1() {}
+
+@abi(@Sendable func sendable2())
+func sendable2() {}
+
+@abi(func sendable3())
+@Sendable func sendable3() {}
+
+// @preconcurrency -- allowed to vary
+@abi(@preconcurrency func preconcurrency1())
+@preconcurrency func preconcurrency1() {}
+
+@abi(@preconcurrency func preconcurrency2())
+func preconcurrency2() {}
+
+@abi(func preconcurrency3())
+@preconcurrency func preconcurrency3() {}
+
+// @_preInverseGenerics -- allowed to vary
+struct PreInverseGenerics<T: ~Copyable> {
+  @abi(@_preInverseGenerics func fn1(_: consuming T))
+  @_preInverseGenerics func fn1(_: consuming T) {}
+
+  @abi(@_preInverseGenerics func fn2(_: consuming T))
+  func fn2(_: consuming T) {}
+
+  @abi(func fn3(_: consuming T))
+  @_preInverseGenerics func fn3(_: consuming T) {}
+}
+
+// 'nonisolated', 'isolated' arguments, global actors -- allowed to vary
+@abi(@MainActor func isolation1())
+@MainActor func isolation1() {}
+
+@abi(func isolation2())
+@MainActor func isolation2() {}
+
+@abi(@MainActor func isolation3())
+func isolation3() {}
+
+@abi(nonisolated func isolation4())
+@MainActor func isolation4() {}
+
+@abi(@MainActor func isolation5())
+nonisolated func isolation5() {}
+
+@abi(func isolation6(_: isolated some Actor))
+@MainActor func isolation6(_: some Actor) {}
+
+@abi(func isolation7(_: some Actor))
+func isolation7(_: isolated some Actor) {}
+
+@abi(@execution(concurrent) func isolation8() async)
+@execution(concurrent) func isolation8() async {}
+
+@abi(func isolation9() async)
+@execution(concurrent) func isolation9() async {}
+
+@abi(@execution(concurrent) func isolation10() async)
+func isolation10() async {}
+
+@abi(nonisolated func isolation11() async)
+@execution(concurrent) func isolation11() async {}
+
+@abi(@execution(concurrent) func isolation12() async)
+nonisolated func isolation12() async {}
+
+@abi(@execution(caller) func isolation13() async)
+@execution(caller) func isolation13() async {}
+
+@abi(func isolation14() async)
+@execution(caller) func isolation14() async {}
+
+@abi(@execution(caller) func isolation15() async)
+func isolation15() async {}
+
+@abi(nonisolated func isolation16() async)
+@execution(caller) func isolation16() async {}
+
+@abi(@execution(caller) func isolation17() async)
+nonisolated func isolation17() async {}
+
+@abi(@execution(caller) func isolation18() async)
+@execution(concurrent) func isolation18() async {}
+
+@abi(@execution(concurrent) func isolation19() async)
+@execution(caller) func isolation19() async {}
+
+// NSCopying - see attr/attr_abi_objc.swift
+
+// @LLDBDebuggerFunction -- banned in @abi
+@abi(@LLDBDebuggerFunction func lldbDebuggerFunction1()) // expected-error {{unused 'LLDBDebuggerFunction' attribute in '@abi'}} {{6-27=}}
+@LLDBDebuggerFunction func lldbDebuggerFunction1() {}
+
+@abi(@LLDBDebuggerFunction func lldbDebuggerFunction2()) // expected-error {{unused 'LLDBDebuggerFunction' attribute in '@abi'}} {{6-27=}}
+func lldbDebuggerFunction2() {}
+
+@abi(func lldbDebuggerFunction3())
+@LLDBDebuggerFunction func lldbDebuggerFunction3() {}
+
+// @_compilerInitialized -- banned in @abi
+class CompilerInitialized {
+  @abi(@_compilerInitialized let v1: Int) // expected-error {{unused '_compilerInitialized' attribute in '@abi'}} {{8-29=}}
+  @_compilerInitialized let v1: Int
+
+  @abi(@_compilerInitialized let v2: Int) // expected-error {{unused '_compilerInitialized' attribute in '@abi'}} {{8-29=}}
+  let v2: Int
+
+  @abi(let v3: Int)
+  @_compilerInitialized let v3: Int
+
+  init() {}
+}
+
+// @_hasStorage -- banned in @abi
+struct HasStorage {
+  @abi(@_hasStorage let v1: Int) // expected-error {{unused '_hasStorage' attribute in '@abi'}} {{8-20=}}
+  @_hasStorage let v1: Int
+
+  @abi(@_hasStorage let v2: Int) // expected-error {{unused '_hasStorage' attribute in '@abi'}} {{8-20=}}
+  let v2: Int
+
+  @abi(let v3: Int)
+  @_hasStorage let v3: Int
+
+  init() {}
+}
+
+// @discardableResult -- banned in @abi
+@abi(@discardableResult func discardableResult1() -> Int) // expected-error {{unused 'discardableResult' attribute in '@abi'}} {{6-24=}}
+@discardableResult func discardableResult1() -> Int { 0 }
+
+@abi(@discardableResult func discardableResult2() -> Int) // expected-error {{unused 'discardableResult' attribute in '@abi'}} {{6-24=}}
+func discardableResult2() -> Int { 0 }
+
+@abi(func lldbDebuggerFunction3() -> Int)
+@discardableResult func discardableResult3() -> Int { 0 }
+
+// @warn_unqualified_access -- banned in @abi
+struct WarnUnqualifiedAccess {
+  @abi(@warn_unqualified_access func fn1()) // expected-error {{unused 'warn_unqualified_access' attribute in '@abi'}} {{8-32=}}
+  @warn_unqualified_access func fn1() {}
+
+  @abi(@warn_unqualified_access func fn2()) // expected-error {{unused 'warn_unqualified_access' attribute in '@abi'}} {{8-32=}}
+  func fn2() {}
+
+  @abi(func fn3())
+  @warn_unqualified_access func fn3() {}
+}
+
+// @_disfavoredOverload -- banned in @abi
+@abi(@_disfavoredOverload func disfavoredOverload1()) // expected-error {{unused '_disfavoredOverload' attribute in '@abi'}} {{6-26=}}
+@_disfavoredOverload func disfavoredOverload1() {}
+
+@abi(@_disfavoredOverload func disfavoredOverload2()) // expected-error {{unused '_disfavoredOverload' attribute in '@abi'}} {{6-26=}}
+func disfavoredOverload2() {}
+
+@abi(func disfavoredOverload3())
+@_disfavoredOverload func disfavoredOverload3() {}
+
+// @_nonEphemeral -- banned in @abi
+@abi(func nonEphemeral1(@_nonEphemeral _: UnsafeRawPointer)) // expected-error {{unused '_nonEphemeral' attribute in '@abi'}} {{25-39=}}
+func nonEphemeral1(@_nonEphemeral _: UnsafeRawPointer) {}
+
+@abi(func nonEphemeral2(@_nonEphemeral _: UnsafeRawPointer)) // expected-error {{unused '_nonEphemeral' attribute in '@abi'}} {{25-39=}}
+func nonEphemeral2(_: UnsafeRawPointer) {}
+
+@abi(func disfavoredOverload3(_: UnsafeRawPointer))
+func nonEphemeral3(@_nonEphemeral _: UnsafeRawPointer) {}
+
+// @_inheritActorContext -- banned in @abi
+@abi(func inheritActorContext1(@_inheritActorContext fn: @Sendable @escaping () async -> Void)) // expected-error {{unused '_inheritActorContext' attribute in '@abi'}} {{32-53=}}
+func inheritActorContext1(@_inheritActorContext fn: @Sendable @escaping () async -> Void) {}
+
+@abi(func inheritActorContext2(@_inheritActorContext fn: @Sendable @escaping () async -> Void)) // expected-error {{unused '_inheritActorContext' attribute in '@abi'}} {{32-53=}}
+func inheritActorContext2(fn: @Sendable @escaping () async -> Void) {}
+
+@abi(func inheritActorContext3(fn: @Sendable @escaping () async -> Void))
+func inheritActorContext3(@_inheritActorContext fn: @Sendable @escaping () async -> Void) {}
+
+// @excusivity(checked/unchecked) -- banned in @abi
+class Exclusivity {
+  @abi(var checked00: Int)
+  var checked00: Int = 0
+
+  @abi(@exclusivity(checked) var checked10: Int) // expected-error {{unused 'exclusivity(checked)' attribute in '@abi'}} {{8-29=}}
+  var checked10: Int = 0
+
+  @abi(@exclusivity(unchecked) var checked20: Int) // expected-error {{unused 'exclusivity(unchecked)' attribute in '@abi'}} {{8-31=}}
+  var checked20: Int = 0
+
+  @abi(var checked01: Int)
+  @exclusivity(checked) var checked01: Int = 0
+
+  @abi(@exclusivity(checked) var checked11: Int) // expected-error {{unused 'exclusivity(checked)' attribute in '@abi'}} {{8-29=}}
+  @exclusivity(checked) var checked11: Int = 0
+
+  @abi(@exclusivity(unchecked) var checked21: Int) // expected-error {{unused 'exclusivity(unchecked)' attribute in '@abi'}} {{8-31=}}
+  @exclusivity(checked) var checked21: Int = 0
+
+  @abi(var checked02: Int)
+  @exclusivity(unchecked) var checked02: Int = 0
+
+  @abi(@exclusivity(checked) var checked12: Int) // expected-error {{unused 'exclusivity(checked)' attribute in '@abi'}} {{8-29=}}
+  @exclusivity(unchecked) var checked12: Int = 0
+
+  @abi(@exclusivity(unchecked) var checked22: Int) // expected-error {{unused 'exclusivity(unchecked)' attribute in '@abi'}} {{8-31=}}
+  @exclusivity(unchecked) var checked22: Int = 0
+}
+
+// @_noAllocation -- banned in @abi
+@abi(@_noAllocation func noAllocation1()) // expected-error {{unused '_noAllocation' attribute in '@abi'}} {{6-20=}}
+@_noAllocation func noAllocation1() {}
+
+@abi(@_noAllocation func noAllocation2()) // expected-error {{unused '_noAllocation' attribute in '@abi'}} {{6-20=}}
+func noAllocation2() {}
+
+@abi(func noAllocation3())
+@_noAllocation func noAllocation3() {}
+
+// @_noLocks -- banned in @abi
+@abi(@_noLocks func noLocks1()) // expected-error {{unused '_noLocks' attribute in '@abi'}} {{6-15=}}
+@_noLocks func noLocks1() {}
+
+@abi(@_noLocks func noLocks2()) // expected-error {{unused '_noLocks' attribute in '@abi'}} {{6-15=}}
+func noLocks2() {}
+
+@abi(func noLocks3())
+@_noLocks func noLocks3() {}
+
+// @_noImplicitCopy -- banned in @abi
+struct NoImplicitCopy {
+  @abi(@_noImplicitCopy func fn1()) // expected-error {{unused '_noImplicitCopy' attribute in '@abi'}} {{8-24=}}
+  @_noImplicitCopy func fn1() {}
+
+  @abi(@_noImplicitCopy func fn2()) // expected-error {{unused '_noImplicitCopy' attribute in '@abi'}} {{8-24=}}
+  func fn2() {}
+
+  @abi(func fn3())
+  @_noImplicitCopy func fn3() {}
+
+  @abi(func fn4(@_noImplicitCopy _: Int)) // expected-error {{unused '_noImplicitCopy' attribute in '@abi'}} {{17-33=}}
+  func fn4(@_noImplicitCopy _: Int) {}
+
+  @abi(func fn5(@_noImplicitCopy _: Int)) // expected-error {{unused '_noImplicitCopy' attribute in '@abi'}} {{17-33=}}
+  func fn5(_: Int) {}
+
+  @abi(func fn6(_: Int))
+  func fn6(@_noImplicitCopy _: Int) {}
+}
+
+// @_noObjCBridging -- banned in @abi
+@abi(@_noObjCBridging func noObjCBridging1()) // expected-error {{unused '_noObjCBridging' attribute in '@abi'}} {{6-22=}}
+@_noObjCBridging func noObjCBridging1() {}
+
+@abi(@_noObjCBridging func noObjCBridging2()) // expected-error {{unused '_noObjCBridging' attribute in '@abi'}} {{6-22=}}
+func noObjCBridging2() {}
+
+@abi(func noObjCBridging3())
+@_noObjCBridging func noObjCBridging3() {}
+
+// @_noExistentials -- banned in @abi
+@abi(@_noExistentials func noExistentials1()) // expected-error {{unused '_noExistentials' attribute in '@abi'}} {{6-22=}}
+@_noExistentials func noExistentials1() {}
+
+@abi(@_noExistentials func noExistentials2()) // expected-error {{unused '_noExistentials' attribute in '@abi'}} {{6-22=}}
+func noExistentials2() {}
+
+@abi(func noExistentials3())
+@_noExistentials func noExistentials3() {}
+
+// @_noRuntime -- banned in @abi
+@abi(@_noRuntime func noRuntime1()) // expected-error {{unused '_noRuntime' attribute in '@abi'}} {{6-17=}}
+@_noRuntime func noRuntime1() {}
+
+@abi(@_noRuntime func noRuntime2()) // expected-error {{unused '_noRuntime' attribute in '@abi'}} {{6-17=}}
+func noRuntime2() {}
+
+@abi(func noRuntime3())
+@_noRuntime func noRuntime3() {}
+
+// @_noEagerMove -- banned in @abi
+struct NoEagerMove {
+  @abi(@_noEagerMove func fn1()) // expected-error {{unused '_noEagerMove' attribute in '@abi'}} {{8-21=}}
+  @_noEagerMove func fn1() {}
+
+  @abi(@_noEagerMove func fn2()) // expected-error {{unused '_noEagerMove' attribute in '@abi'}} {{8-21=}}
+  func fn2() {}
+
+  @abi(func fn3())
+  @_noEagerMove func fn3() {}
+
+  @abi(func fn4(@_noEagerMove _: Int)) // expected-error {{unused '_noEagerMove' attribute in '@abi'}} {{17-30=}}
+  func fn4(@_noEagerMove _: Int) {}
+
+  @abi(func fn5(@_noEagerMove _: Int)) // expected-error {{unused '_noEagerMove' attribute in '@abi'}} {{17-30=}}
+  func fn5(_: Int) {}
+
+  @abi(func fn6(_: Int))
+  func fn6(@_noEagerMove _: Int) {}
+}
+
+// @_eagerMove -- banned in @abi
+struct EagerMove {
+  @abi(@_eagerMove func fn1()) // expected-error {{unused '_eagerMove' attribute in '@abi'}} {{8-19=}}
+  @_eagerMove func fn1() {}
+
+  @abi(@_eagerMove func fn2()) // expected-error {{unused '_eagerMove' attribute in '@abi'}} {{8-19=}}
+  func fn2() {}
+
+  @abi(func fn3())
+  @_eagerMove func fn3() {}
+
+  @abi(func fn4(@_eagerMove _: Int)) // expected-error {{unused '_eagerMove' attribute in '@abi'}} {{17-28=}}
+  func fn4(@_eagerMove _: Int) {}
+
+  @abi(func fn5(@_eagerMove _: Int)) // expected-error {{unused '_eagerMove' attribute in '@abi'}} {{17-28=}}
+  func fn5(_: Int) {}
+
+  @abi(func fn6(_: Int))
+  func fn6(@_eagerMove _: Int) {}
+}
+
+// @_lexicalLifetimes -- banned in @abi
+@abi(@_lexicalLifetimes func lexicalLifetimes1()) // expected-error {{unused '_lexicalLifetimes' attribute in '@abi'}} {{6-24=}}
+@_lexicalLifetimes func lexicalLifetimes1() {}
+
+@abi(@_lexicalLifetimes func lexicalLifetimes2()) // expected-error {{unused '_lexicalLifetimes' attribute in '@abi'}} {{6-24=}}
+func lexicalLifetimes2() {}
+
+@abi(func lexicalLifetimes3())
+@_lexicalLifetimes func lexicalLifetimes3() {}
+
+// @_assemblyVision -- banned in @abi
+@abi(@_assemblyVision func assemblyVision1()) // expected-error {{unused '_assemblyVision' attribute in '@abi'}} {{6-22=}}
+@_assemblyVision func assemblyVision1() {}
+
+@abi(@_assemblyVision func assemblyVision2()) // expected-error {{unused '_assemblyVision' attribute in '@abi'}} {{6-22=}}
+func assemblyVision2() {}
+
+@abi(func assemblyVision3())
+@_assemblyVision func assemblyVision3() {}
+
+// @_extern -- banned in @abi
+@abi(@_extern(c) @_extern(wasm, module: "foo", name: "bar") func extern1()) // expected-error {{unused '_extern' attribute in '@abi'}} {{18-61=}}  expected-error {{unused '_extern' attribute in '@abi'}} {{6-17=}}
+@_extern(c) @_extern(wasm, module: "foo", name: "bar") func extern1()
+
+@abi(@_extern(c) @_extern(wasm, module: "foo", name: "bar") func extern2()) // expected-error {{unused '_extern' attribute in '@abi'}} {{18-61=}} expected-error {{unused '_extern' attribute in '@abi'}} {{6-17=}}
+func extern2() {}
+
+@abi(func extern3())
+@_extern(c) @_extern(wasm, module: "foo", name: "bar") func extern3()
+
+// @_used -- banned in @abi
+@abi(@_used func used1()) // expected-error {{unused '_used' attribute in '@abi'}} {{6-12=}}
+@_used func used1() {}
+
+@abi(@_used func used2()) // expected-error {{unused '_used' attribute in '@abi'}} {{6-12=}}
+func used2() {}
+
+@abi(func used3())
+@_used func used3() {}
+
+// weak, unowned, unowned(unsafe) -- banned in @abi
+class ReferenceOwnership {
+  @abi(var v00: AnyObject?)
+  var v00: AnyObject? = nil
+
+  @abi(weak var v10: AnyObject?) // expected-error {{unused 'weak' modifier in '@abi'}} {{8-12=}}
+  var v10: AnyObject? = nil
+
+  @abi(unowned var v20: AnyObject?) // expected-error {{unused 'unowned' modifier in '@abi'}} {{8-15=}}
+  var v20: AnyObject? = nil
+
+  @abi(unowned(unsafe) var v30: AnyObject?) // expected-error {{unused 'unowned(unsafe)' modifier in '@abi'}} {{8-23=}}
+  var v30: AnyObject? = nil
+
+  @abi(var v01: AnyObject?)
+  weak var v01: AnyObject? = nil
+
+  @abi(weak var v11: AnyObject?) // expected-error {{unused 'weak' modifier in '@abi'}} {{8-12=}}
+  weak var v11: AnyObject? = nil
+
+  @abi(unowned var v21: AnyObject?) // expected-error {{unused 'unowned' modifier in '@abi'}} {{8-15=}}
+  weak var v21: AnyObject? = nil
+
+  @abi(unowned(unsafe) var v31: AnyObject?) // expected-error {{unused 'unowned(unsafe)' modifier in '@abi'}} {{8-23=}}
+  weak var v31: AnyObject? = nil
+
+  @abi(var v02: AnyObject?)
+  unowned var v02: AnyObject? = nil
+
+  @abi(weak var v12: AnyObject?) // expected-error {{unused 'weak' modifier in '@abi'}} {{8-12=}}
+  unowned var v12: AnyObject? = nil
+
+  @abi(unowned var v22: AnyObject?) // expected-error {{unused 'unowned' modifier in '@abi'}} {{8-15=}}
+  unowned var v22: AnyObject? = nil
+
+  @abi(unowned(unsafe) var v32: AnyObject?) // expected-error {{unused 'unowned(unsafe)' modifier in '@abi'}} {{8-23=}}
+  unowned var v32: AnyObject? = nil
+
+  @abi(var v03: AnyObject?)
+  unowned(unsafe) var v03: AnyObject? = nil
+
+  @abi(weak var v13: AnyObject?) // expected-error {{unused 'weak' modifier in '@abi'}} {{8-12=}}
+  unowned(unsafe) var v13: AnyObject? = nil
+
+  @abi(unowned var v23: AnyObject?) // expected-error {{unused 'unowned' modifier in '@abi'}} {{8-15=}}
+  unowned(unsafe) var v23: AnyObject? = nil
+
+  @abi(unowned(unsafe) var v33: AnyObject?) // expected-error {{unused 'unowned(unsafe)' modifier in '@abi'}} {{8-23=}}
+  unowned(unsafe) var v33: AnyObject? = nil
+}
+
+// @abi -- banned in @abi (no recursion)
+@abi(
+  @abi(func abiRecursion()) // expected-error {{unused 'abi' attribute in '@abi'}} {{3-29=}}
+  func abiRecursion()
+)
+func abiRecursion() {}
+
+// @unsafe -- banned in @abi
+@abi(@unsafe func unsafe1()) // expected-error {{unused 'unsafe' attribute in '@abi'}} {{6-13=}}
+@unsafe func unsafe1() {}
+
+@abi(@unsafe func unsafe2()) // expected-error {{unused 'unsafe' attribute in '@abi'}} {{6-13=}}
+func unsafe2() {}
+
+@abi(func unsafe3())
+@unsafe func unsafe3() {}
+
+// @safe -- banned in @abi
+@abi(@safe func safe1()) // expected-error {{unused 'safe' attribute in '@abi'}} {{6-11=}}
+@safe func safe1() {}
+
+@abi(@safe func safe2()) // expected-error {{unused 'safe' attribute in '@abi'}} {{6-11=}}
+func safe2() {}
+
+@abi(func safe3())
+@safe func safe3() {}
+
+// Access control, @usableFromInline, @_spi -- banned in @abi
+// An ABI-only decl gets its access control from its counterpart.
+@abi(internal func accessControl1()) // expected-error {{unused 'internal' modifier in '@abi'}} {{6-14=}}
+func accessControl1() {}
+
+@abi(func accessControl2())
+public func accessControl2() {}
+
+@abi(@usableFromInline func accessControl3()) // expected-error {{'@usableFromInline' attribute can only be applied to internal or package declarations, but global function 'accessControl3()' is public}}
+public func accessControl3() {}
+
+@abi(private(set) var setterAccess1: Int) // expected-error {{unused 'private' modifier in '@abi'}} {{6-18=}}
+var setterAccess1: Int = 42
+
+@abi(var setterAccess2: Int)
+private(set) var setterAccess2: Int = 42
+
+@abi(@usableFromInline func usableFromInline1()) // expected-error {{unused 'usableFromInline' attribute in '@abi'}} {{6-23=}}
+@usableFromInline func usableFromInline1() {}
+
+@abi(func usableFromInline2())
+@usableFromInline func usableFromInline2() {}
+
+@_spi(foo) public struct SPIType {} // expected-note 2 {{struct declared here}}
+
+@abi(@_spi(foo) func spi1(_: SPIType)) // expected-error {{unused '_spi' attribute in '@abi'}} {{6-16=}}
+@_spi(foo) public func spi1(_: SPIType) {}
+
+@abi(func spi2(_: SPIType))
+@_spi(foo) public func spi2(_: SPIType) {}
+
+@abi(func spi3(_: SPIType)) // expected-error {{cannot use struct 'SPIType' here; it is SPI}}
+public func spi3(_: SPIType) {} // expected-error {{cannot use struct 'SPIType' here; it is SPI}}
+
+// @available, @_unavailable*, @backDeployed -- banned in @abi 
+// An ABI-only decl gets its availability from its counterpart.
+@abi(@available(macOS 14, iOS 16, *) func available1()) // expected-error {{unused 'available' attribute in '@abi'}} {{6-37=}}
+@available(macOS 14, iOS 16, *) func available1() {}
+
+@abi(@available(macOS 14, iOS 16, *) func available2()) // expected-error {{unused 'available' attribute in '@abi'}} {{6-37=}}
+func available2() {}
+
+@abi(func available3())
+@available(macOS 14, iOS 16, *) func available3() {}
+
+@abi(
+  @available(macOS, unavailable) // expected-error {{unused 'available' attribute in '@abi'}} {{3-34=}}
+  @available(iOS, deprecated) // expected-error {{unused 'available' attribute in '@abi'}} {{3-31=}}
+  func available4()
+)
+@available(macOS 14, iOS 16, *) func available4() {}
+
+// Additional tests in attr/attr_abi_objc.swift
+
+@abi(@_unavailableFromAsync func unavailableFromAsync1()) // expected-error {{unused '_unavailableFromAsync' attribute in '@abi'}} {{6-28=}}
+@_unavailableFromAsync func unavailableFromAsync1() {}
+
+@abi(@_unavailableFromAsync func unavailableFromAsync2()) // expected-error {{unused '_unavailableFromAsync' attribute in '@abi'}} {{6-28=}}
+func unavailableFromAsync2() {}
+
+@abi(func unavailableFromAsync3())
+@_unavailableFromAsync func unavailableFromAsync3() {}
+
+// FIXME: Test @_unavailableInEmbedded (it gets rewritten in the parser)
+
+@abi(@backDeployed(before: macOS 14) func backDeployed1()) // expected-error {{unused 'backDeployed' attribute in '@abi'}} {{6-37=}}
+@backDeployed(before: macOS 14) public func backDeployed1() {}
+
+@abi(@backDeployed(before: macOS 14) func backDeployed2()) // expected-error {{unused 'backDeployed' attribute in '@abi'}} {{6-37=}}
+public func backDeployed2() {}
+
+@abi(func backDeployed3())
+@backDeployed(before: macOS 14) public func backDeployed3() {}
+
+// override, @_nonoverride -- banned in @abi
+// An ABI-only decl gets its overrides from its counterpart; no marker modifiers
+// are required.
+class Overridden {
+  func fn1() {}
+
+  func fn2() {} // expected-note 2 {{overridden declaration is here}}
+
+  func fn3() {}
+}
+
+class Override: Overridden {
+  @abi(override func fn1()) // expected-error {{unused 'override' modifier in '@abi'}} {{8-16=}}
+  override func fn1() {}
+
+  @abi(override func fn2()) // expected-error {{unused 'override' modifier in '@abi'}} {{8-16=}}
+  func fn2() {} // expected-error {{overriding declaration requires an 'override' keyword}}
+
+  @abi(func fn3())
+  override func fn3() {}
+}
+
+class NonOverride: Overridden {
+  @abi(@_nonoverride func fn1()) // expected-error {{unused '_nonoverride' attribute in '@abi'}} {{8-21=}}
+  @_nonoverride func fn1() {}
+
+  @abi(@_nonoverride func fn2()) // expected-error {{unused '_nonoverride' attribute in '@abi'}} {{8-21=}}
+  func fn2() {} // expected-error {{overriding declaration requires an 'override' keyword}}
+
+  @abi(func fn3())
+  @_nonoverride func fn3() {}
+}
+
+// @_silgen_name -- banned in @abi *and* on declarations with @abi
+// Becuase of the way @_silgen_name is implemented, these would interact oddly
+// if they were allowed on the same decl.
+@_silgen_name("conflictingAttrsSilgenName")
+@abi(func silgenName1())
+func silgenName1() {} // expected-error@-2 {{cannot use '@_silgen_name' and '@abi' on the same global function because they serve the same purpose}} {{1-44=}}
+
+@abi(@_silgen_name("silgenNameWithABI") func silgenName2()) // expected-error {{unused '_silgen_name' attribute in '@abi'}} {{6-40=}}
+func silgenName2() {}
+
+// @_documentation(visibility:metadata:) -- banned in @abi
+@abi(@_documentation(visibility: public) func documentation1()) // expected-error {{unused '_documentation' attribute in '@abi'}} {{6-41=}}
+@_documentation(visibility: public) func documentation1() {}
+
+@abi(@_documentation(visibility: public) func documentation2()) // expected-error {{unused '_documentation' attribute in '@abi'}} {{6-41=}}
+func documentation2() {}
+
+@abi(func documentation3())
+@_documentation(visibility: public) func documentation3() {}
+
+// @_allowFeatureSuppression -- banned in @abi
+// Feature suppression should be applied to the API since we can't put `#if`
+// inside `@abi`.
+@abi(@_allowFeatureSuppression(IsolatedAny) func allowFeatureSuppression1()) // expected-error {{unused '_allowFeatureSuppression' attribute in '@abi'}} {{6-44=}}
+@_allowFeatureSuppression(IsolatedAny) func allowFeatureSuppression1() {}
+
+@abi(@_allowFeatureSuppression(IsolatedAny) func allowFeatureSuppression2()) // expected-error {{unused '_allowFeatureSuppression' attribute in '@abi'}} {{6-44=}}
+func allowFeatureSuppression2() {}
+
+@abi(func allowFeatureSuppression3())
+@_allowFeatureSuppression(IsolatedAny) func allowFeatureSuppression3() {}
+
+// @objc -- tested in attr/attr_abi_objc.swift
+// @IBAction -- tested in attr/attr_abi_objc.swift
+// @IBInspectable -- tested in attr/attr_abi_objc.swift
+// @GKInspectable -- tested in attr/attr_abi_objc.swift
+// @IBOutlet -- tested in attr/attr_abi_objc.swift
+// @IBSegueAction -- tested in attr/attr_abi_objc.swift
+// @NSManaged -- tested in attr/attr_abi_objc.swift
+// @nonobjc -- tested in attr/attr_abi_objc.swift
+// optional -- tested in attr/attr_abi_objc.swift
+// dynamic -- tested in attr/attr_abi_objc.swift
+
+// @_cdecl -- banned in @abi
+// ABI-only decls inherit cdecl-ness from their counterpart
+@abi(@_cdecl("cdecl1") func cdecl1()) // expected-error {{unused '_cdecl' attribute in '@abi'}} {{6-23=}}
+@_cdecl("cdecl1") func cdecl1() {}
+
+@abi(@_cdecl("cdecl2") func cdecl2()) // expected-error {{unused '_cdecl' attribute in '@abi'}} {{6-23=}}
+func cdecl2() {}
+
+@abi(func cdecl3())
+@_cdecl("cdecl3") func cdecl3() {}
+
+// @implementation -- banned in @abi
+// ABI-only decls inherit implementation-ness from their counterpart
+@abi(@implementation func implementation1()) // expected-error {{unused 'implementation' attribute in '@abi'}} {{6-21=}}
+@_cdecl("implementation1") @implementation func implementation1() {}
+
+@abi(@implementation func implementation2()) // expected-error {{unused 'implementation' attribute in '@abi'}} {{6-21=}}
+@_cdecl("implementation2") func implementation2() {}
+
+@abi(func implementation3())
+@_cdecl("implementation3") @implementation func implementation3() {}
+
+// @_expose -- banned in @abi
+// ABI-only decls inherit exposure from their counterpart
+@abi(@_expose(Cxx) func expose1()) // expected-error {{unused '_expose' attribute in '@abi'}} {{6-19=}}
+@_expose(Cxx) func expose1() {}
+
+@abi(@_expose(Cxx) func expose2()) // expected-error {{unused '_expose' attribute in '@abi'}} {{6-19=}}
+func expose2() {}
+
+@abi(func expose3())
+@_expose(Cxx) func expose3() {}
+
+// @_section -- banned in @abi
+@abi(@_section("fnord") func section1()) // expected-error {{unused '_section' attribute in '@abi'}} {{6-24=}}
+@_section("fnord") func section1() {}
+
+@abi(@_section("fnord") func section2()) // expected-error {{unused '_section' attribute in '@abi'}} {{6-24=}}
+func section2() {}
+
+@abi(func section3())
+@_section("fnord") func section3() {}
+
+// @inlinable -- automatically cloned into @abi
+@abi(@inlinable func inlinable1())
+@inlinable func inlinable1() {}
+
+@abi(@inlinable func inlinable2()) // expected-error {{extra 'inlinable' attribute in '@abi'}} {{6-16=}}
+func inlinable2() {}
+
+@abi(func inlinable3()) // expected-remark {{inferred '@inlinable' in '@abi' to match attribute on API}}
+@inlinable func inlinable3() {} // expected-note {{matches attribute here}}
+
+// @inline -- automatically cloned into @abi
+@abi(@inline(never) func inline1())
+@inline(never) func inline1() {}
+
+@abi(@inline(never) func inline2()) // expected-error {{extra 'inline(never)' attribute in '@abi'}} {{6-20=}}
+func inline2() {}
+
+@abi(func inline3()) // expected-remark {{inferred '@inline(never)' in '@abi' to match attribute on API}}
+@inline(never) func inline3() {} // expected-note {{matches attribute here}}
+
+// @_transparent -- automatically cloned into @abi
+@abi(@_transparent func transparent1())
+@_transparent func transparent1() {}
+
+@abi(@_transparent func transparent2()) // expected-error {{extra '_transparent' attribute in '@abi'}} {{6-19=}}
+func transparent2() {}
+
+@abi(func transparent3()) // expected-remark {{inferred '@_transparent' in '@abi' to match attribute on API}}
+@_transparent func transparent3() {} // expected-note {{matches attribute here}}
+
+// @_alwaysEmitIntoClient -- automatically cloned into @abi
+@abi(@_alwaysEmitIntoClient func alwaysEmitIntoClient1())
+@_alwaysEmitIntoClient func alwaysEmitIntoClient1() {}
+
+@abi(@_alwaysEmitIntoClient func alwaysEmitIntoClient2()) // expected-error {{extra '_alwaysEmitIntoClient' attribute in '@abi'}} {{6-28=}}
+func alwaysEmitIntoClient2() {}
+
+@abi(func alwaysEmitIntoClient3()) // expected-remark {{inferred '@_alwaysEmitIntoClient' in '@abi' to match attribute on API}}
+@_alwaysEmitIntoClient func alwaysEmitIntoClient3() {} // expected-note {{matches attribute here}}
+
+// @_optimize(none) -- banned in @abi
+@abi(@_optimize(none) func optimize1()) // expected-error {{unused '_optimize(none)' attribute in '@abi'}} {{6-22=}}
+@_optimize(none) func optimize1() {}
+
+@abi(@_optimize(none) func optimize2()) // expected-error {{unused '_optimize(none)' attribute in '@abi'}} {{6-22=}}
+func optimize2() {}
+
+@abi(func optimize3())
+@_optimize(none) func optimize3() {}
+
+// convenience -- must match in @abi
+// This doesn't have direct mangling impact, but a future direction where
+// convenience inits could fake designated inits or vice versa might be useful.
+class Convenience {
+  @abi(convenience init(i1: Void))
+  convenience init(i1: Void) { fatalError() }
+
+  @abi(convenience init(i2: Void))  // expected-error {{extra 'convenience' modifier in '@abi'}} {{8-19=}}
+  init(i2: Void) { fatalError() }
+
+  @abi(init(i3: Void)) // expected-error {{missing 'convenience' modifier in '@abi'}} {{8-8=convenience }}
+  convenience init(i3: Void) { fatalError() } // expected-note {{should match modifier here}}
+}
+
+// required -- must match in @abi
+// This doesn't have direct mangling impact, but a future direction where
+// required inits could fake normal inits or vice versa might be useful.
+class Required {
+  @abi(required init(i1: Void))
+  required init(i1: Void) { fatalError() }
+
+  @abi(required init(i2: Void))  // expected-error {{extra 'required' modifier in '@abi'}} {{8-16=}}
+  init(i2: Void) { fatalError() }
+
+  @abi(init(i3: Void)) // expected-error {{missing 'required' modifier in '@abi'}} {{8-8=required }}
+  required init(i3: Void) { fatalError() } // expected-note {{should match modifier here}}
+}
+
+// lazy -- automatically cloned into @abi
+class Lazy {
+  @abi(lazy var v1: Int)
+  lazy var v1: Int = 0
+
+  @abi(lazy var v2: Int) // expected-error {{extra 'lazy' modifier in '@abi'}} {{8-12=}}
+  var v2: Int = 0
+
+  @abi(var v3: Int) // expected-remark {{inferred 'lazy' in '@abi' to match modifier on API}}
+  lazy var v3: Int = 0 // expected-note {{matches modifier here}}
+}
+
+// @_fixed_layout -- banned in @abi
+class FixedLayoutVars {
+  @abi(@_fixed_layout var v1: Int) // expected-error {{unused '_fixed_layout' attribute in '@abi'}} {{8-22=}}
+  @_fixed_layout public var v1: Int = 0
+
+  @abi(@_fixed_layout var v2: Int) // expected-error {{unused '_fixed_layout' attribute in '@abi'}} {{8-22=}}
+  public var v2: Int = 0
+
+  @abi(var v3: Int)
+  @_fixed_layout public var v3: Int = 0
+}
+
+// @_specialize -- banned in @abi
+// TODO: Maybe use @_specialize in @abi to tweak the ABI of specializations.
+// Ban it for now, since there's nothing useful you can do with it yet.
+@abi(@_specialize(where T == Int) func specialize1<T>(_: T)) // expected-error {{unused '_specialize' attribute in '@abi'}} {{6-34=}}
+@_specialize(where T == Int) func specialize1<T>(_: T) {}
+
+@abi(@_specialize(where T == Int) func specialize2<T>(_: T)) // expected-error {{unused '_specialize' attribute in '@abi'}} {{6-34=}}
+func specialize2<T>(_: T) {}
+
+@abi(func specialize3<T>(_: T))
+@_specialize(where T == Int) func specialize3<T>(_: T) {}
+
+// @_effects -- banned in @abi
+@abi(@_effects(readonly) func effects1()) // expected-error {{unused '_effects(readonly)' attribute in '@abi'}} {{6-25=}}
+@_effects(readonly) func effects1() {}
+
+@abi(@_effects(readonly) func effects2()) // expected-error {{unused '_effects(readonly)' attribute in '@abi'}} {{6-25=}}
+func effects2() {}
+
+@abi(func effects3())
+@_effects(readonly) func effects3() {}
+
+// @_implements -- banned in @abi
+protocol ImplementsProto {
+  func f1()
+  func f2()
+  func f3()
+}
+
+class Implements: ImplementsProto {
+  @abi(@_implements(ImplementsProto, f1) func f1()) // expected-error {{unused '_implements' attribute in '@abi'}} {{8-41=}}
+  @_implements(ImplementsProto, f1) func f1() {}
+
+  @abi(@_implements(ImplementsProto, f2) func f2()) // expected-error {{unused '_implements' attribute in '@abi'}} {{8-41=}}
+  func f2() {}
+
+  @abi(func f3())
+  @_implements(ImplementsProto, f3) func f3() {}
+}
+
+// @_dynamicReplacement -- banned in @abi
+struct DynamicReplacement {
+  dynamic func f1Original() {}
+  dynamic func f2Original() {}
+  dynamic func f3Original() {}
+}
+
+extension DynamicReplacement {
+  @abi(@_dynamicReplacement(for: f1Original) func f1()) // expected-error {{unused '_dynamicReplacement' attribute in '@abi'}} {{8-45=}}
+  @_dynamicReplacement(for: f1Original) func f1() {}
+
+  @abi(@_dynamicReplacement(for: f2Original) func f2()) // expected-error {{unused '_dynamicReplacement' attribute in '@abi'}} {{8-45=}}
+  func f2() {}
+
+  @abi(func f3())
+  @_dynamicReplacement(for: f3Original) func f3() {}
+}
+
+// @_weakLinked -- tested in attr/attr_weaklinked.swift
+
+// @_borrowed -- automatically cloned into @abi
+protocol BorrowedAttr {
+  @abi(@_borrowed var v1: Int)
+  @_borrowed var v1: Int { get set }
+
+  @abi(var v2: Int) // expected-remark {{inferred '@_borrowed' in '@abi' to match attribute on API}}
+  @_borrowed var v2: Int { get set } // expected-note {{matches attribute here}}
+
+  @abi(@_borrowed var v3: Int) // expected-error {{extra '_borrowed' attribute in '@abi'}} {{8-18=}}
+  var v3: Int { get set }
+}
+
+// @lifetime -- must match in @abi
+// TODO: Probably possible to make these unconstrained as long as we ensure
+// that `@_addressableForDependencies` doesn't cause a calling convention
+// change.
+struct Lifetime: ~Escapable {
+  @abi(@lifetime(borrow i1) init(i1: UnsafeRawPointer))
+  @lifetime(borrow i1) init(i1: UnsafeRawPointer) {}
+
+  @abi(@lifetime(borrow i2) init(i2: UnsafeRawPointer)) // expected-error {{extra 'lifetime' attribute in '@abi'}} {{8-28=}}
+  init(i2: UnsafeRawPointer) {}
+
+  @abi(init(i3: UnsafeRawPointer)) // expected-error {{missing 'lifetime' attribute in '@abi'}} {{8-8=@lifetime(borrow i3) }}
+  @lifetime(borrow i3) init(i3: UnsafeRawPointer) {} // expected-note {{should match attribute here}}
+
+  @abi(@lifetime(borrow i4) init(i4: UnsafeRawPointer, i4a: UnsafeRawPointer)) // expected-error {{'lifetime' attribute in '@abi' should match '@lifetime(borrow i4a)'}} {{8-28=@lifetime(borrow i4a)}}
+  @lifetime(borrow i4a) init(i4: UnsafeRawPointer, i4a: UnsafeRawPointer) {} // expected-note {{should match attribute here}}
+}
+
+// @_unsafeNonescapableResult -- must match in @abi
+// TODO: This could probably be allowed to vary in some circumstances.
+struct UnsafeNonescapableResult: ~Escapable {
+  @abi(@_unsafeNonescapableResult init(i1: UnsafeRawPointer))
+  @_unsafeNonescapableResult init(i1: UnsafeRawPointer) {}
+
+  @abi(@_unsafeNonescapableResult init(i2: UnsafeRawPointer)) // expected-error {{extra '_unsafeNonescapableResult' attribute in '@abi'}} {{8-34=}}
+  init(i2: UnsafeRawPointer) {}
+
+  @abi(init(i3: UnsafeRawPointer)) // expected-error {{missing '_unsafeNonescapableResult' attribute in '@abi'}} {{8-8=@_unsafeNonescapableResult }}
+  @_unsafeNonescapableResult init(i3: UnsafeRawPointer) {} // expected-note {{should match attribute here}}
+}
+
+// distributed -- must match in @abi
+@available(SwiftStdlib 5.7, *)
+distributed actor Local {
+  @abi(distributed func fn1())
+  distributed func fn1() {}
+
+  @abi(distributed func fn2()) // expected-error {{extra 'distributed' modifier in '@abi'}} {{8-19=}}
+  func fn2() {}
+
+  @abi(func fn3()) // expected-error {{missing 'distributed' modifier in '@abi'}} {{8-8=distributed }}
+  distributed func fn3() {} // expected-note {{should match modifier here}}
+}
+
+// _const -- allowed to vary
+@abi(func const1(_: _const Int))
+func const1(_: _const Int) {}
+
+@abi(func const2(_: _const Int))
+func const2(_: Int) {}
+
+@abi(func const3(_: Int))
+func const3(_: _const Int) {}
+
+// @derivative, @differentiable, @transpose, @_noDerivative -- banned in @abi
+// Too complex to infer or check
+// TODO: Figure out if there's something we could do here.
+@abi(@differentiable(reverse) func differentiable1(_ x: Float) -> Float) // expected-error {{unused 'differentiable' attribute in '@abi'}} {{6-30=}}
+@differentiable(reverse) func differentiable1(_ x: Float) -> Float { x }
+
+@abi(@differentiable(reverse) func differentiable2(_ x: Float) -> Float) // expected-error {{unused 'differentiable' attribute in '@abi'}} {{6-30=}}
+func differentiable2(_ x: Float) -> Float { x }
+
+@abi(func differentiable3(_ x: Float) -> Float)
+@differentiable(reverse) func differentiable3(_ x: Float) -> Float { x }
+
+@abi(
+  @derivative(of: differentiable1(_:)) // expected-error {{unused 'derivative' attribute in '@abi'}} {{3-40=}}
+  func derivative1(_: Float) -> (value: Float, differential: (Float) -> (Float))
+)
+@derivative(of: differentiable1(_:))
+func derivative1(_ x: Float) -> (value: Float, differential: (Float) -> (Float)) {
+  return (x, { $0 })
+}
+
+@abi(
+  @derivative(of: differentiable2(_:)) // expected-error {{unused 'derivative' attribute in '@abi'}} {{3-40=}}
+  func derivative2(_: Float) -> (value: Float, differential: (Float) -> (Float))
+)
+func derivative2(_ x: Float) -> (value: Float, differential: (Float) -> (Float)) {
+  return (x, { $0 })
+}
+
+@abi(
+  func derivative3(_: Float) -> (value: Float, differential: (Float) -> (Float))
+)
+@derivative(of: differentiable3(_:))
+func derivative3(_ x: Float) -> (value: Float, differential: (Float) -> (Float)) {
+  return (x, { $0 })
+}
+
+struct Transpose<T: Differentiable & AdditiveArithmetic> where T == T.TangentVector {
+  func fn1(_ x: T, _ y: T) -> T { x + y }
+  func fn2(_ x: T, _ y: T) -> T { x + y }
+  func fn3(_ x: T, _ y: T) -> T { x + y }
+
+  @abi(
+    @transpose(of: fn1, wrt: (0, 1)) // expected-error {{unused 'transpose' attribute in '@abi'}} {{5-38=}}
+    func t_fn1(_ result: T) -> (T, T)
+  )
+  @transpose(of: fn1, wrt: (0, 1))
+  func t_fn1(_ result: T) -> (T, T) { (result, result) }
+
+  @abi(
+    @transpose(of: fn2, wrt: (0, 1)) // expected-error {{unused 'transpose' attribute in '@abi'}} {{5-38=}}
+    func t_fn2(_ result: T) -> (T, T)
+  )
+  func t_fn2(_ result: T) -> (T, T) { (result, result) }
+
+  @abi(
+    func t_fn3(_ result: T) -> (T, T)
+  )
+  @transpose(of: fn3, wrt: (0, 1))
+  func t_fn3(_ result: T) -> (T, T) { (result, result) }
+}
+
+struct NoDerivative {
+  @abi(@noDerivative func fn1()) // expected-error {{unused 'noDerivative' attribute in '@abi'}} {{8-21=}}
+  @noDerivative func fn1() {}
+
+  @abi(@noDerivative func fn2()) // expected-error {{unused 'noDerivative' attribute in '@abi'}} {{8-21=}}
+  func fn2() {}
+
+  @abi(func fn3())
+  @noDerivative func fn3() {}
+}
+
+// prefix, postfix -- allowed to vary
+// Essentially part of the name, which is unconstrained.
+prefix operator ←
+prefix operator ↑ // expected-note {{prefix operator found here}}
+prefix operator → // expected-note {{prefix operator found here}}
+prefix operator ↓
+
+struct Prefix {
+  @abi(static prefix func ← (value: Self) -> Self)
+  static prefix func ← (value: Self) -> Self { value }
+
+  @abi(static prefix func ↑ (value: Self) -> Self)
+  static func ↑ (value: Self) -> Self { value } // expected-error {{prefix unary operator missing 'prefix' modifier}}
+
+  @abi(static func → (value: Self) -> Self) // expected-error {{prefix unary operator missing 'prefix' modifier}}
+  static prefix func → (value: Self) -> Self { value }
+
+  // Test ABI-preserving replacement code pattern:
+
+  @abi(static prefix func ↓ (value: Self) -> Self)
+  static func __oldDownArrow(value: Self) -> Self { value }
+
+  @abi(static func __newDownArrow(value: Self) -> Self)
+  static prefix func ↓ (value: Self) -> Self { value }
+}
+
+postfix operator ←←
+postfix operator ↑↑ // expected-note {{postfix operator found here}}
+postfix operator →→ // expected-note {{postfix operator found here}}
+postfix operator ↓↓
+
+struct Postfix {
+  @abi(static postfix func ←← (value: Self) -> Self)
+  static postfix func ←← (value: Self) -> Self { value }
+
+  @abi(static postfix func ↑↑ (value: Self) -> Self)
+  static func ↑↑ (value: Self) -> Self { value } // expected-error {{postfix unary operator missing 'postfix' modifier}}
+
+  @abi(static func →→ (value: Self) -> Self) // expected-error {{postfix unary operator missing 'postfix' modifier}}
+  static postfix func →→ (value: Self) -> Self { value }
+
+  // Test ABI-preserving replacement code pattern:
+
+  @abi(static postfix func ↓↓ (value: Self) -> Self)
+  static func __oldDownArrow(value: Self) -> Self { value }
+
+  @abi(static func __newDownArrow(value: Self) -> Self)
+  static postfix func ↓↓ (value: Self) -> Self { value }
+}
+
+// Not testing `infix`; it's not *really* valid on funcs.
+
+// nonmutating, borrowing, consuming, __consuming, mutating -- allowed to vary
+// Act like param modifiers; checked against each other separately
+struct SelfParamOwnership {
+  @abi(func fn00())
+  func fn00() {}
+
+  @abi(nonmutating func fn10())
+  func fn10() {}
+
+  @abi(borrowing func fn20())
+  func fn20() {}
+
+  @abi(consuming func fn30()) // expected-error {{modifier 'consuming' on self parameter in '@abi' is not compatible with default}} {{none}}
+  func fn30() {} // expected-note {{should match modifier here}}
+
+  @abi(__consuming func fn40()) // expected-error {{modifier '__consuming' on self parameter in '@abi' is not compatible with default}} {{none}}
+  func fn40() {} // expected-note {{should match modifier here}}
+
+  @abi(mutating func fn50()) // expected-error {{modifier 'mutating' on self parameter in '@abi' is not compatible with default}} {{none}}
+  func fn50() {} // expected-note {{should match modifier here}}
+
+  @abi(func fn01())
+  nonmutating func fn01() {}
+
+  @abi(nonmutating func fn11())
+  nonmutating func fn11() {}
+
+  @abi(borrowing func fn21())
+  nonmutating func fn21() {}
+
+  @abi(consuming func fn31()) // expected-error {{modifier 'consuming' on self parameter in '@abi' is not compatible with default}} {{none}}
+  nonmutating func fn31() {} // expected-note {{should match modifier here}}
+
+  @abi(__consuming func fn41()) // expected-error {{modifier '__consuming' on self parameter in '@abi' is not compatible with default}} {{none}}
+  nonmutating func fn41() {} // expected-note {{should match modifier here}}
+
+  @abi(mutating func fn51()) // expected-error {{modifier 'mutating' on self parameter in '@abi' is not compatible with default}} {{none}}
+  nonmutating func fn51() {} // expected-note {{should match modifier here}}
+
+  @abi(func fn02())
+  borrowing func fn02() {}
+
+  @abi(nonmutating func fn12())
+  borrowing func fn12() {}
+
+  @abi(borrowing func fn22())
+  borrowing func fn22() {}
+
+  @abi(consuming func fn32()) // expected-error {{modifier 'consuming' on self parameter in '@abi' is not compatible with 'borrowing'}} {{none}}
+  borrowing func fn32() {} // expected-note {{should match modifier here}}
+
+  @abi(__consuming func fn42()) // expected-error {{modifier '__consuming' on self parameter in '@abi' is not compatible with 'borrowing'}} {{none}}
+  borrowing func fn42() {} // expected-note {{should match modifier here}}
+
+  @abi(mutating func fn52()) // expected-error {{modifier 'mutating' on self parameter in '@abi' is not compatible with 'borrowing'}} {{none}}
+  borrowing func fn52() {} // expected-note {{should match modifier here}}
+
+  @abi(func fn03()) // expected-error {{default modifier on self parameter in '@abi' is not compatible with 'consuming'}} {{none}}
+  consuming func fn03() {} // expected-note {{should match modifier here}}
+
+  @abi(nonmutating func fn13()) // expected-error {{default modifier on self parameter in '@abi' is not compatible with 'consuming'}} {{none}}
+  consuming func fn13() {} // expected-note {{should match modifier here}}
+
+  @abi(borrowing func fn23()) // expected-error {{modifier 'borrowing' on self parameter in '@abi' is not compatible with 'consuming'}} {{none}}
+  consuming func fn23() {} // expected-note {{should match modifier here}}
+
+  @abi(consuming func fn33())
+  consuming func fn33() {}
+
+  @abi(__consuming func fn43())
+  consuming func fn43() {}
+
+  @abi(mutating func fn53()) // expected-error {{modifier 'mutating' on self parameter in '@abi' is not compatible with 'consuming'}} {{none}}
+  consuming func fn53() {} // expected-note {{should match modifier here}}
+
+  @abi(func fn04()) // expected-error {{default modifier on self parameter in '@abi' is not compatible with '__consuming'}} {{none}}
+  __consuming func fn04() {} // expected-note {{should match modifier here}}
+
+  @abi(nonmutating func fn14()) // expected-error {{default modifier on self parameter in '@abi' is not compatible with '__consuming'}} {{none}}
+  __consuming func fn14() {} // expected-note {{should match modifier here}}
+
+  @abi(borrowing func fn24()) // expected-error {{modifier 'borrowing' on self parameter in '@abi' is not compatible with '__consuming'}} {{none}}
+  __consuming func fn24() {} // expected-note {{should match modifier here}}
+
+  @abi(consuming func fn34())
+  __consuming func fn34() {}
+
+  @abi(__consuming func fn44())
+  __consuming func fn44() {}
+
+  @abi(mutating func fn54()) // expected-error {{modifier 'mutating' on self parameter in '@abi' is not compatible with '__consuming'}} {{none}}
+  __consuming func fn54() {} // expected-note {{should match modifier here}}
+
+  @abi(func fn05()) // expected-error {{default modifier on self parameter in '@abi' is not compatible with 'mutating'}} {{none}}
+  mutating func fn05() {} // expected-note {{should match modifier here}}
+
+  @abi(nonmutating func fn15()) // expected-error {{default modifier on self parameter in '@abi' is not compatible with 'mutating'}} {{none}}
+  mutating func fn15() {} // expected-note {{should match modifier here}}
+
+  @abi(borrowing func fn25()) // expected-error {{modifier 'borrowing' on self parameter in '@abi' is not compatible with 'mutating'}} {{none}}
+  mutating func fn25() {} // expected-note {{should match modifier here}}
+
+  @abi(consuming func fn35()) // expected-error {{modifier 'consuming' on self parameter in '@abi' is not compatible with 'mutating'}} {{none}}
+  mutating func fn35() {} // expected-note {{should match modifier here}}
+
+  @abi(__consuming func fn45()) // expected-error {{modifier '__consuming' on self parameter in '@abi' is not compatible with 'mutating'}} {{none}}
+  mutating func fn45() {} // expected-note {{should match modifier here}}
+
+  @abi(mutating func fn55())
+  mutating func fn55() {}
+}
+
+// @_addressableSelf -- act like type attribute on `self`
+struct AddressableSelf {
+  @abi(@_addressableSelf func fn1())
+  @_addressableSelf func fn1() {}
+
+  @abi(@_addressableSelf func fn2()) // expected-error {{attribute '_addressableSelf' on self parameter in '@abi' is not compatible with default}} {{none}}
+  func fn2() {} // expected-note {{should match attribute here}}
+
+  @abi(func fn3()) // expected-error {{default attribute on self parameter in '@abi' is not compatible with '_addressableSelf'}} {{none}}
+  @_addressableSelf func fn3() {} // expected-note {{should match attribute here}}
+}
+
 //
 // Incorrect usage
 //
diff --git a/test/attr/attr_abi_objc.swift b/test/attr/attr_abi_objc.swift
new file mode 100644
index 0000000000000..581d1e5ab358a
--- /dev/null
+++ b/test/attr/attr_abi_objc.swift
@@ -0,0 +1,156 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-feature ABIAttribute -parse-as-library -Rabi-inference
+
+// REQUIRES: swift_feature_ABIAttribute
+// REQUIRES: objc_interop
+
+import Foundation
+
+// @NSCopying -- banned in @abi
+class NSCopyingAttr: NSObject {
+  @abi(@NSCopying var v1: NSArray?) // expected-error {{unused 'NSCopying' attribute in '@abi'}} {{8-18=}}
+  @NSCopying var v1: NSArray? = nil
+
+  @abi(var v2: NSArray?)
+  @NSCopying var v2: NSArray? = nil
+}
+
+// Availability
+// These tests will only work on a versioned platform.
+@available(macOS 99, iOS 99, tvOS 99, watchOS 99, visionOS 99, *)
+struct FutureType {}
+
+@abi(func available5(_: FutureType)) // expected-error {{'FutureType' is only available in }}
+func available5(_: FutureType) {} // expected-error {{'FutureType' is only available in }}
+// expected-note@-1 2{{add @available attribute to enclosing global function}} (fix-it not tested because it varies by target)
+
+@abi(func available6(_: FutureType))
+@available(macOS 99, iOS 99, tvOS 99, watchOS 99, visionOS 99, *) func available6(_: FutureType) {}
+
+// @objc -- banned in @abi
+class ObjCAttr: NSObject {
+  @abi(@objc func fn1()) // expected-error {{unused 'objc' attribute in '@abi'}} {{8-13=}}
+  @objc func fn1() {}
+
+  @abi(@objc func fn2()) // expected-error {{unused 'objc' attribute in '@abi'}} {{8-13=}}
+  func fn2() {}
+
+  @abi(func fn3())
+  @objc func fn3() {}
+}
+
+// @IBAction -- banned in @abi
+class IBActionAttr: NSObject {
+  @abi(@IBAction func fn1(_: Any)) // expected-error {{unused 'IBAction' attribute in '@abi'}} {{8-17=}}
+  @IBAction func fn1(_: Any) {}
+
+  @abi(@IBAction func fn2(_: Any)) // expected-error {{unused 'IBAction' attribute in '@abi'}} {{8-17=}}
+  func fn2(_: Any) {}
+
+  @abi(func fn3(_: Any))
+  @IBAction func fn3(_: Any) {}
+}
+
+// @IBInspectable -- banned in @abi
+class IBInspectableAttr: NSObject {
+  @abi(@IBInspectable var v1: Double) // expected-error {{unused 'IBInspectable' attribute in '@abi'}} {{8-22=}}
+  @IBInspectable var v1: Double = 0.0
+
+  @abi(@IBInspectable var v2: Double) // expected-error {{unused 'IBInspectable' attribute in '@abi'}} {{8-22=}}
+  var v2: Double = 0.0
+
+  @abi(var v3: Double)
+  @IBInspectable var v3: Double = 0.0
+}
+
+// @GKInspectable -- banned in @abi
+class GKInspectableAttr: NSObject {
+  @abi(@GKInspectable var v1: Double) // expected-error {{unused 'GKInspectable' attribute in '@abi'}} {{8-22=}}
+  @GKInspectable var v1: Double = 0.0
+
+  @abi(@GKInspectable var v2: Double) // expected-error {{unused 'GKInspectable' attribute in '@abi'}} {{8-22=}}
+  var v2: Double = 0.0
+
+  @abi(var v3: Double)
+  @GKInspectable var v3: Double = 0.0
+}
+
+// @IBOutlet -- banned in @abi
+class IBOutletAttr: NSObject {
+  @abi(@IBOutlet var v1: NSObject!) // expected-error {{unused 'IBOutlet' attribute in '@abi'}} {{8-17=}}
+  @IBOutlet var v1: NSObject!
+
+  @abi(@IBOutlet var v2: NSObject!) // expected-error {{unused 'IBOutlet' attribute in '@abi'}} {{8-17=}}
+  var v2: NSObject!
+
+  @abi(var v3: NSObject!)
+  @IBOutlet var v3: NSObject!
+}
+
+// @IBSegueAction -- banned in @abi
+class IBSegueActionAttr: NSObject {
+  @abi(@IBSegueAction func fn1(_: Any) -> Any) // expected-error {{unused 'IBSegueAction' attribute in '@abi'}} {{8-22=}}
+  @IBSegueAction func fn1(_: Any) -> Any {}
+
+  @abi(@IBSegueAction func fn2(_: Any) -> Any) // expected-error {{unused 'IBSegueAction' attribute in '@abi'}} {{8-22=}}
+  func fn2(_: Any) -> Any {}
+
+  @abi(func fn3(_: Any) -> Any)
+  @IBSegueAction func fn3(_: Any) -> Any {}
+}
+
+// @NSManaged -- banned in @abi
+class NSManagedAttr: NSObject {
+  @abi(@NSManaged var v1: NSObject!) // expected-error {{unused 'NSManaged' attribute in '@abi'}} {{8-18=}}
+  @NSManaged var v1: NSObject!
+
+  @abi(@NSManaged var v2: NSObject!) // expected-error {{unused 'NSManaged' attribute in '@abi'}} {{8-18=}}
+  var v2: NSObject!
+
+  @abi(var v3: NSObject!)
+  @NSManaged var v3: NSObject!
+}
+
+// @nonobjc -- banned in @abi
+@objcMembers
+class NonObjCAttr: NSObject {
+  @abi(@nonobjc var v1: NSObject!) // expected-error {{unused 'nonobjc' attribute in '@abi'}} {{8-16=}}
+  @nonobjc var v1: NSObject!
+
+  @abi(@nonobjc var v2: NSObject!) // expected-error {{unused 'nonobjc' attribute in '@abi'}} {{8-16=}}
+  var v2: NSObject!
+
+  @abi(var v3: NSObject!)
+  @nonobjc var v3: NSObject!
+}
+
+// optional -- banned in @abi
+@objc protocol OptionalModifier {
+  @abi(
+    @objc // expected-error {{unused 'objc' attribute in '@abi'}} {{5-11=}}
+    optional // expected-error {{unused 'optional' modifier in '@abi'}} {{5-14=}}
+    func fn1()
+  )
+  @objc optional func fn1()
+
+  @abi(
+    @objc // expected-error {{unused 'objc' attribute in '@abi'}} {{5-11=}}
+    optional // expected-error {{unused 'optional' modifier in '@abi'}} {{5-14=}}
+    func fn2()
+  )
+  @objc func fn2()
+
+  @abi(func fn3())
+  @objc optional func fn3()
+}
+
+// dynamic -- banned in @abi
+class DynamicModifier: NSObject {
+  @abi(dynamic func fn1()) // expected-error {{unused 'dynamic' modifier in '@abi'}} {{8-15=}}
+  dynamic func fn1() {}
+
+  @abi(dynamic func fn2()) // expected-error {{unused 'dynamic' modifier in '@abi'}} {{8-15=}}
+  func fn2() {}
+
+  @abi(func fn3())
+  dynamic func fn3() {}
+}
diff --git a/test/attr/attr_weaklinked.swift b/test/attr/attr_weaklinked.swift
index 2de0a8eaddbb3..a56dbf4bf8ecb 100644
--- a/test/attr/attr_weaklinked.swift
+++ b/test/attr/attr_weaklinked.swift
@@ -1,6 +1,18 @@
-// RUN: %target-typecheck-verify-swift
+// RUN: %target-typecheck-verify-swift -enable-experimental-feature ABIAttribute
+
+// UNSUPPORTED: OS=windows-msvc
+// REQUIRES: swift_feature_ABIAttribute
 
-#if !os(Windows)
 @_weakLinked public func f() { }
-#endif
 
+// @_weakLinked -- banned in @abi
+struct WeakLinked {
+  @abi(@_weakLinked func fn1()) // expected-error {{unused '_weakLinked' attribute in '@abi'}} {{8-20=}}
+  @_weakLinked func fn1() {}
+
+  @abi(@_weakLinked func fn2()) // expected-error {{unused '_weakLinked' attribute in '@abi'}} {{8-20=}}
+  func fn2() {}
+
+  @abi(func fn3())
+  @_weakLinked func fn3() {}
+}
diff --git a/tools/SourceKit/lib/SwiftLang/SwiftEditor.cpp b/tools/SourceKit/lib/SwiftLang/SwiftEditor.cpp
index cb0ff4f8f47c2..6a7e6067c7c51 100644
--- a/tools/SourceKit/lib/SwiftLang/SwiftEditor.cpp
+++ b/tools/SourceKit/lib/SwiftLang/SwiftEditor.cpp
@@ -1454,9 +1454,8 @@ class SwiftDocumentStructureWalker: public ide::SyntaxModelWalker {
     // We only report runtime name for classes and protocols with an explicitly
     // defined ObjC name, i.e. those that have @objc("SomeName")
     if (D && (isa<ClassDecl>(D) || isa<ProtocolDecl>(D))) {
-      auto *ObjCNameAttr = D->getAttrs().getAttribute<ObjCAttr>();
-      if (ObjCNameAttr && ObjCNameAttr->hasName())
-        return ObjCNameAttr->getName()->getString(Buf);
+      if (auto objcName = D->getExplicitObjCName())
+        return objcName->getString(Buf);
     }
     return StringRef();
   }