[clang] Fix issues on template class enum member redeclaration

Author: thebrandre

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -1627,12 +1627,17 @@ Decl *TemplateDeclInstantiator::VisitEnumDecl(EnumDecl *D) {
   // specialization causes the implicit instantiation of the declarations, but
   // not the definitions of scoped member enumerations.
   //
-  // DR1484 clarifies that enumeration definitions inside of a template
+  // DR1484 clarifies that enumeration definitions inside a template
   // declaration aren't considered entities that can be separately instantiated
-  // from the rest of the entity they are declared inside of.
+  // from the rest of the entity they are declared inside.
   if (isDeclWithinFunction(D) ? D == Def : Def && !Enum->isScoped()) {
-    SemaRef.CurrentInstantiationScope->InstantiatedLocal(D, Enum);
-    InstantiateEnumDefinition(Enum, Def);
+    // Prevent redundant instantiation of the enumerator-definition if the
+    // definition has already been instantiated due to a prior
+    // opaque-enum-declaration.
+    if (PrevDecl == nullptr) {
+      SemaRef.CurrentInstantiationScope->InstantiatedLocal(D, Enum);
+      InstantiateEnumDefinition(Enum, Def);
+    }
   }
 
   return Enum;