Allow redeclaration of __declspec(uuid)

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

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D71439

Author: Zachary Henkel

clang/lib/Sema/SemaDecl.cpp

@@ -2685,6 +2685,10 @@ static void checkNewAttributesAfterDef(Sema &S, Decl *New, const Decl *Old) {
       // 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:

clang/lib/Sema/SemaDeclAttr.cpp

@@ -5394,9 +5394,11 @@ UuidAttr *Sema::mergeUuidAttr(Decl *D, const AttributeCommonInfo &CI,
   if (const auto *UA = D->getAttr<UuidAttr>()) {
     if (UA->getGuid().equals_lower(Uuid))
       return nullptr;
-    Diag(UA->getLocation(), diag::err_mismatched_uuid);
-    Diag(CI.getLoc(), diag::note_previous_uuid);
-    D->dropAttr<UuidAttr>();
+    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);

clang/test/SemaCXX/ms-uuid.cpp

@@ -106,3 +106,9 @@ void F2() {
 }
 
 }
+
+// Test class/struct redeclaration where the subsequent
+// declaration has a uuid attribute
+struct X{};
+
+struct __declspec(uuid("00000000-0000-0000-0000-000000000000")) X;