PR45063: Fix crash on invalid processing an elaborated class template-id

with an invalid scope specifier.

(cherry picked from commit 44c3a63)

Author: zygoloid

clang/lib/Sema/SemaTemplate.cpp

@@ -3817,6 +3817,9 @@ TypeResult Sema::ActOnTagTemplateIdType(TagUseKind TUK,
                                         SourceLocation LAngleLoc,
                                         ASTTemplateArgsPtr TemplateArgsIn,
                                         SourceLocation RAngleLoc) {
+  if (SS.isInvalid())
+    return TypeResult(true);
+
   TemplateName Template = TemplateD.get();
 
   // Translate the parser's template argument list in our AST format.

clang/test/Parser/cxx-template-decl.cpp

@@ -269,3 +269,7 @@ namespace AnnotateAfterInvalidTemplateId {
 
   void f() { A<0, 0>::f(); } // expected-error {{ambiguous partial specializations}}
 }
+
+namespace PR45063 {
+  template<class=class a::template b<>> struct X {}; // expected-error {{undeclared identifier 'a'}}
+}