Allow redeclaration of __declspec(uuid)

[zahen]

Summary

msvc allows a subsequent declaration of a uuid attribute on a struct/class. Mirror this behavior in clang-cl.

[rnk]

Seems reasonable, looks good to me.

[zahen]

@rnk Can you please submit on my behalf. I don't have commit access.

[rnk]

In D71439#1789141, @zahen wrote:

@rnk Can you please submit on my behalf. I don't have commit access.

Sorry for the delay, I pushed it today. Thanks!

clang/lib/Sema/SemaDecl.cpp

if (hasAttribute(Def, NewAttribute->getKind())) {
       ++I;
       continue; // regular attr merging will take care of validating this.
     }
 
     if (isa<C11NoReturnAttr>(NewAttribute)) {
       // C's _Noreturn is allowed to be added to a function after it is defined.
       ++I;
       continue;
+    } else if (isa<UuidAttr>(NewAttribute)) {
+      // msvc will allow a subsequent definition to add an uuid to a class
+      ++I;
+      continue;
     } else if (const AlignedAttr *AA = dyn_cast<AlignedAttr>(NewAttribute)) {
       if (AA->isAlignas()) {
         // C++11 [dcl.align]p6:
         //   if any declaration of an entity has an alignment-specifier,
         //   every defining declaration of that entity shall specify an
         //   equivalent alignment.
         // C11 6.7.5/7:
         //   If the definition of an object does not have an alignment

clang/lib/Sema/SemaDeclAttr.cpp

 // Microsoft specific attribute handlers.
 //===----------------------------------------------------------------------===//
 
 UuidAttr *Sema::mergeUuidAttr(Decl *D, const AttributeCommonInfo &CI,
                               StringRef Uuid) {
   if (const auto *UA = D->getAttr<UuidAttr>()) {
     if (UA->getGuid().equals_lower(Uuid))
       return nullptr;
+    if (!UA->getGuid().empty()) {
       Diag(UA->getLocation(), diag::err_mismatched_uuid);
       Diag(CI.getLoc(), diag::note_previous_uuid);
       D->dropAttr<UuidAttr>();
     }
+  }
 
   return ::new (Context) UuidAttr(Context, CI, Uuid);
 }
 
 static void handleUuidAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   if (!S.LangOpts.CPlusPlus) {
     S.Diag(AL.getLoc(), diag::err_attribute_not_supported_in_lang)
         << AL << AttributeLangSupport::C;

clang/test/SemaCXX/ms-uuid.cpp

 }
 
 void F2() {
   // The UUID should work for a non-type template parameter.
   F1<&__uuidof(C1)>();
 }
 
 }
+
+// Test class/struct redeclaration where the subsequent
+// declaration has a uuid attribute
+struct X{};
+
+struct __declspec(uuid("00000000-0000-0000-0000-000000000000")) X;
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