[clang] Fix missing promotion type for opaque enum declarations in class templates

Repeat the steps in `Sema::ActOnTag` for the promotion type after type substitution of the
underlying type. This is required in the case of an *opaque-enum-declaration* (see [dcl.enum]).
If, instead, a full *enum-specifier* (subsequent curly braces) is provided, `Sema::ActOnEnumBody`
is re-invoked on template instantiation overriding the promotion type and hiding the issue.
Note that, in contrast to `Sema::ActOnEnumBody`, `Sema::ActOnTag` is *not* called again for the
instantiated enumeration type.

Fixes #116525.

Author: thebrandre

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -1571,6 +1571,23 @@ Decl *TemplateDeclInstantiator::VisitEnumDecl(EnumDecl *D) {
         Enum->setIntegerType(SemaRef.Context.IntTy);
       else
         Enum->setIntegerTypeSourceInfo(NewTI);
+
+      // The following lines are relevant for opaque-enum-declarations.
+      // If users declare a full enum-specifier, the promotion type is reset
+      // again when parsing the (potentially empty) enumerator-list.
+      // We implement the same logic from C++11 [conv.prom] p4 here but only
+      // after template specialization [temp.spec.general] because it requires
+      // the concrete type.
+      // Note that (1) this also correctly handles the non-dependent case,
+      // (2) we set the promotion type for scoped enumerations to ensure
+      // consistency with declarations outside of templates, (3) we guarantee a
+      // valid promotion even if the user provided an invalid underlying type
+      // (fallback to "default int").
+      QualType UnderlyingType = Enum->getIntegerType();
+      Enum->setPromotionType(
+          SemaRef.Context.isPromotableIntegerType(UnderlyingType)
+              ? SemaRef.Context.getPromotedIntegerType(UnderlyingType)
+              : UnderlyingType);
     } else {
       assert(!D->getIntegerType()->isDependentType()
              && "Dependent type without type source info");