Sema: Relax enum parameter pack restriction for CodingKeys

lib/AST/Decl.cpp

@@ -6363,6 +6363,9 @@ ConstructorDecl *NominalTypeDecl::getDefaultInitializer() const {
 }
 
 void NominalTypeDecl::synthesizeSemanticMembersIfNeeded(DeclName member) {
+  if (isa<ProtocolDecl>(this))
+    return;
+
   // Silently break cycles here because we can't be sure when and where a
   // request to synthesize will come from yet.
   // FIXME: rdar://56844567

lib/Sema/CodeSynthesis.cpp

@@ -1364,6 +1364,8 @@ evaluator::SideEffect
 ResolveImplicitMemberRequest::evaluate(Evaluator &evaluator,
                                        NominalTypeDecl *target,
                                        ImplicitMemberAction action) const {
+  ASSERT(!isa<ProtocolDecl>(target));
+
   // FIXME: This entire request is a layering violation made of smaller,
   // finickier layering violations. See rdar://56844567
 

lib/Sema/TypeCheckDecl.cpp

@@ -2941,7 +2941,7 @@ static ArrayRef<Decl *> evaluateMembersRequest(
     }
   }
 
-  if (nominal) {
+  if (nominal && !isa<ProtocolDecl>(nominal)) {
     // If the type conforms to Encodable or Decodable, even via an extension,
     // the CodingKeys enum is synthesized as a member of the type itself.
     // Force it into existence.

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -3226,11 +3226,13 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
 
     // Temporary restriction until we figure out pattern matching and
     // enum case construction with packs.
-    if (auto genericSig = ED->getGenericSignature()) {
-      for (auto paramTy : genericSig.getGenericParams()) {
-        if (paramTy->isParameterPack()) {
-          ED->diagnose(diag::enum_with_pack);
-          break;
+    if (!ED->isSynthesized()) {
+      if (auto genericSig = ED->getGenericSignature()) {
+        for (auto paramTy : genericSig.getGenericParams()) {
+          if (paramTy->isParameterPack()) {
+            ED->diagnose(diag::enum_with_pack);
+            break;
+          }
         }
       }
     }

test/decl/protocol/special/coding/codable_parameter_pack.swift

@@ -0,0 +1,7 @@
+// RUN: %target-typecheck-verify-swift -target %target-swift-5.9-abi-triple
+
+// We should accept this:
+
+public struct HasPack<each T>: Codable {
+  var x: String?
+}