Remove qualtype qualifier in coroutine error to prevent assert in debug

Summary:
A forward-declared coroutine_traits should trip an error; we need
a complete type.

Unfortunately, in debug mode only, we trip an assert when attempting
to provide the fully qualified type for the error message.
If you try to compile a program with a forward-declared
coroutine_traits in debug mode, clang will crash.

I've included a test for the behavior and removed the q modifier
on the error message. This prevents the crash in debug mode and
does not change the behavior for the error message on a
forward-declaration of a coroutine_traits type.

Test Plan:
I've included a test for the forward-declaration.

Patch by Tanoy Sinha!

Reviewers: modocache, GorNishanov

Reviewed By: modocache

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D49099

llvm-svn: 336748

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -9082,7 +9082,7 @@ def err_coroutine_promise_type_incomplete : Error<
   "this function cannot be a coroutine: %0 is an incomplete type">;
 def err_coroutine_type_missing_specialization : Error<
   "this function cannot be a coroutine: missing definition of "
-  "specialization %q0">;
+  "specialization %0">;
 def err_coroutine_promise_incompatible_return_functions : Error<
   "the coroutine promise type %0 declares both 'return_value' and 'return_void'">;
 def err_coroutine_promise_requires_return_function : Error<

clang/test/SemaCXX/coroutine-traits-undefined-template.cpp

@@ -0,0 +1,21 @@
+// test/SemaCXX/coroutine-traits-undefined-template.cpp
+
+// This file contains references to sections of the Coroutines TS, which can be
+// found at http://wg21.link/coroutines.
+
+// RUN: %clang_cc1 -std=c++14 -fcoroutines-ts -verify %s -fcxx-exceptions -fexceptions -Wunused-result
+
+namespace std {
+namespace experimental {
+
+template<typename ...T>
+struct coroutine_traits {
+  struct promise_type {};
+};
+
+template<> struct coroutine_traits<void>; // expected-note {{forward declaration of 'std::experimental::coroutine_traits<void>'}}
+}} // namespace std::experimental
+
+void uses_forward_declaration() {
+  co_return; // expected-error {{this function cannot be a coroutine: missing definition of specialization 'coroutine_traits<void>'}}
+}