[clang] [clang] Implement CWG2877 "Type-only lookup for using-enum-declarator" (PR #95399)

[Vlad Serebrennikov via cfe-commits]

================
@@ -738,16 +738,47 @@ Parser::DeclGroupPtrTy Parser::ParseUsingDeclaration(
       return nullptr;
     }
 
-    if (!Tok.is(tok::identifier)) {
+    Decl *UED = nullptr;
+
+    if (Tok.is(tok::identifier)) {
+      IdentifierInfo *IdentInfo = Tok.getIdentifierInfo();
+      SourceLocation IdentLoc = ConsumeToken();
+
+      ParsedType Type = Actions.getTypeName(
+          *IdentInfo, IdentLoc, getCurScope(), &SS, /*isClassName=*/true,
+          /*HasTrailingDot=*/false,
+          /*ObjectType=*/nullptr, /*IsCtorOrDtorName=*/false,
+          /*WantNontrivialTypeSourceInfo=*/true);
+
+      UED = Actions.ActOnUsingEnumDeclaration(
+          getCurScope(), AS, UsingLoc, UELoc, IdentLoc, *IdentInfo, Type, &SS);
+    } else if (Tok.is(tok::annot_template_id)) {
+      TemplateIdAnnotation *TemplateId = takeTemplateIdAnnotation(Tok);
+
+      if (TemplateId->mightBeType()) {
+        AnnotateTemplateIdTokenAsType(SS, ImplicitTypenameContext::No,
+                                      /*IsClassName=*/true);
+
+        assert(Tok.is(tok::annot_typename) && "template-id -> type failed");
+        TypeResult Type = getTypeAnnotation(Tok);
+        SourceRange Loc = Tok.getAnnotationRange();
+        ConsumeAnnotationToken();
+
+        UED = Actions.ActOnUsingEnumDeclaration(getCurScope(), AS, UsingLoc,
+                                                UELoc, Loc, *TemplateId->Name,
+                                                Type.get(), &SS);
+      } else {
+        Diag(Tok.getLocation(), diag::err_using_enum_not_enum)
+            << TemplateId->Name->getName()
----------------
Endilll wrote:

This leads to suboptimal diagnostic when e.g. variable template is put after `using enum`: `using enum A<int>; // expected-error {{A is not an enumerated type}}`. I believe we should tell the user what we see instead of a mildly cryptic "enumeration type is expected." Is there a way to get that out of `TemplateIdAnnotation` that we have here?

https://github.com/llvm/llvm-project/pull/95399