[c-interop] Reject @_silgen_name with @_extern

To avoid ambiguity at SIL level name since both tries to change the SIL
level name.

Author: kateinoigakukun

include/swift/AST/DiagnosticsSema.def

@@ -1877,7 +1877,7 @@ ERROR(extern_not_at_top_level_func,none,
 ERROR(extern_empty_c_name,none,
       "expected non-empty C name in @_extern attribute", ())
 ERROR(extern_only_non_other_attr,none,
-      "@_extern attribute cannot be applied to an '%0' declaration", (StringRef))
+      "@_extern attribute cannot be applied to an '@%0' declaration", (StringRef))
 ERROR(c_func_variadic, none,
       "cannot declare variadic argument %0 in %kind1",
       (DeclName, const ValueDecl *))

lib/Sema/TypeCheckAttr.cpp

@@ -2144,9 +2144,14 @@ void AttributeChecker::visitExternAttr(ExternAttr *attr) {
     }
   }
 
-  // @_cdecl cannot be mixed with @_extern since @_cdecl is for definitions
-  if (D->getAttrs().hasAttribute<CDeclAttr>())
-    diagnose(attr->getLocation(), diag::extern_only_non_other_attr, "@_cdecl");
+  for (auto *otherAttr : D->getAttrs()) {
+    // @_cdecl cannot be mixed with @_extern since @_cdecl is for definitions
+    // @_silgen_name cannot be mixed to avoid SIL-level name ambiguity
+    if (isa<CDeclAttr>(otherAttr) || isa<SILGenNameAttr>(otherAttr)) {
+      diagnose(attr->getLocation(), diag::extern_only_non_other_attr,
+               otherAttr->getAttrName());
+    }
+  }
 }
 
 void AttributeChecker::visitUsedAttr(UsedAttr *attr) {

test/attr/attr_extern.swift

@@ -119,3 +119,16 @@ func withAtCDecl_C()
 @_extern(wasm, module: "", name: "") // expected-error {{@_extern attribute cannot be applied to an '@_cdecl' declaration}}
 @_cdecl("another_c_name")
 func withAtCDecl_Wasm()
+
+@_extern(c) // expected-error {{@_extern attribute cannot be applied to an '@_silgen_name' declaration}}
+@_silgen_name("another_sil_name")
+func withAtSILGenName_C()
+
+@_extern(wasm, module: "", name: "") // expected-error {{@_extern attribute cannot be applied to an '@_silgen_name' declaration}}
+@_silgen_name("another_sil_name")
+func withAtSILGenName_Wasm()
+
+@_extern(c) // expected-error {{@_extern attribute cannot be applied to an '@_silgen_name' declaration}} expected-error {{@_extern attribute cannot be applied to an '@_cdecl' declaration}}
+@_cdecl("another_c_name")
+@_silgen_name("another_sil_name")
+func withAtSILGenName_CDecl_C()