Parse: handle another case of invalid handling for attributes

[compnerd]

Summary

clang would improperly disallow GNU attributes before C++ standard
attributes when a declaration had a linkage specifier. Handle this
similarly to the previous case of invalid parsing. We now better match
the parsing rules from GCC.

[aaron.ballman]

LGTM! Should this also get a release note or do you think the current one covers it well enough?

[compnerd]

I think that the current release note should be sufficient. This is handling the same scenario in a different path.

clang/lib/Parse/ParseDeclCXX.cpp

Decl *Parser::ParseLinkage(ParsingDeclSpec &DS, DeclaratorContext Context) {
   Decl *LinkageSpec =
       Lang.isInvalid()
           ? nullptr
           : Actions.ActOnStartLinkageSpecification(
                 getCurScope(), DS.getSourceRange().getBegin(), Lang.get(),
                 Tok.is(tok::l_brace) ? Tok.getLocation() : SourceLocation());
 
   ParsedAttributes DeclAttrs(AttrFactory);
-  MaybeParseCXX11Attributes(DeclAttrs);
-  ParsedAttributes EmptyDeclSpecAttrs(AttrFactory);
+  ParsedAttributes DeclSpecAttrs(AttrFactory);
+
+  while (MaybeParseCXX11Attributes(DeclAttrs) ||
+         MaybeParseGNUAttributes(DeclSpecAttrs))
+    ;
 
   if (Tok.isNot(tok::l_brace)) {
     // Reset the source range in DS, as the leading "extern"
     // does not really belong to the inner declaration ...
     DS.SetRangeStart(SourceLocation());
     DS.SetRangeEnd(SourceLocation());
     // ... but anyway remember that such an "extern" was seen.
     DS.setExternInLinkageSpec(true);
-    ParseExternalDeclaration(DeclAttrs, EmptyDeclSpecAttrs, &DS);
+    ParseExternalDeclaration(DeclAttrs, DeclSpecAttrs, &DS);
     return LinkageSpec ? Actions.ActOnFinishLinkageSpecification(
                              getCurScope(), LinkageSpec, SourceLocation())
                        : nullptr;
   }
 
   DS.abort();
 
   ProhibitAttributes(DeclAttrs);
while (true) {
 
     case tok::r_brace:
       if (!NestedModules)
         break;
       [[fallthrough]];
     default:
       ParsedAttributes DeclAttrs(AttrFactory);
       MaybeParseCXX11Attributes(DeclAttrs);
-      ParseExternalDeclaration(DeclAttrs, EmptyDeclSpecAttrs);
+      ParseExternalDeclaration(DeclAttrs, DeclSpecAttrs);
       continue;
     }
 
     break;
   }
 
   T.consumeClose();
   return LinkageSpec ? Actions.ActOnFinishLinkageSpecification(

clang/test/Parser/cxx-attributes.cpp

 // RUN: %clang_cc1 -fsyntax-only -verify %s
 
 // GH#58229 - rejects-valid
 __attribute__((__visibility__("default"))) [[nodiscard]] int f();
 [[nodiscard]] __attribute__((__visibility__("default"))) int f();
+extern "C" __attribute__((__visibility__("default"))) [[nodiscard]]
+int g() { return 32; }
 
 class c {
   virtual void f1(const char* a, ...)
     __attribute__ (( __format__(__printf__,2,3) )) = 0;
   virtual void f2(const char* a, ...)
     __attribute__ (( __format__(__printf__,2,3) )) {}
 };