Ensure correct diagnostic still gets used

Author: ilovepi

clang/lib/Sema/SemaCXXScopeSpec.cpp

@@ -780,8 +780,11 @@ bool Sema::BuildCXXNestedNameSpecifier(Scope *S, NestedNameSpecInfo &IdInfo,
 
   if (!Found.empty()) {
     const auto *ND = Found.getAsSingle<NamedDecl>();
-    if (!ND)
+    if (!ND) {
+      Diag(IdInfo.IdentifierLoc, diag::err_expected_class_or_namespace)
+          << IdInfo.Identifier << getLangOpts().CPlusPlus;
       return true;
+    }
     if (::ExtendNestedNameSpecifier(*this, SS, ND, IdInfo.IdentifierLoc,
                                     IdInfo.CCLoc)) {
       const Type *T = SS.getScopeRep().getAsType();

clang/test/Sema/PR166843.cpp

@@ -2,6 +2,6 @@
 namespace a {
 template <class b>
 void c() {
-  ((::c::)); // expected-error {{expected unqualified-id}}
+  ((::c::x)); // expected-error {{'c' is not a class, namespace, or enumeration}}
 }
 }