[clang] Allow using std::coroutine_traits in std::experimental

This is that diff I was aiming for.  When transitioning code from
coroutines-ts to c++20, it can be useful to add a using declaration to
std::experimental pointing to std::coroutine_traits.  This permits
that use by checking whether lookup in std::experimentl finds a
different decl to lookup in std.  You still get a warning about
std::experimental::coroutine_traits being a thing, just not an error.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D115943

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -11047,7 +11047,7 @@ def warn_deprecated_coroutine_namespace : Warning<
   "use std::%0 instead">,
   InGroup<DeprecatedExperimentalCoroutine>;
 def err_mixed_use_std_and_experimental_namespace_for_coroutine : Error<
-  "mixed use of std and std::experimental namespaces for "
+  "conflicting mixed use of std and std::experimental namespaces for "
   "coroutine components">;
 def err_implicit_coroutine_std_nothrow_type_not_found : Error<
   "std::nothrow was not found; include <new> before defining a coroutine which "

clang/lib/Sema/SemaCoroutine.cpp

@@ -1761,8 +1761,9 @@ ClassTemplateDecl *Sema::lookupCoroutineTraits(SourceLocation KwLoc,
       auto *Found = *ResExp.begin();
       Diag(Found->getLocation(), diag::note_entity_declared_at) << Found;
 
-      if (InStd) {
-        // Also found in std
+      if (InStd &&
+          StdCoroutineTraitsCache != ResExp.getAsSingle<ClassTemplateDecl>()) {
+        // Also found something different in std
         Diag(KwLoc,
              diag::err_mixed_use_std_and_experimental_namespace_for_coroutine);
         Diag(StdCoroutineTraitsCache->getLocation(),

clang/test/SemaCXX/coroutine-mixed3-exp-namespace.cpp

@@ -0,0 +1,30 @@
+// RUN: %clang_cc1 -verify -std=c++20 -fsyntax-only %s
+
+#include "Inputs/std-coroutine.h"
+
+namespace std::experimental {
+using std::coroutine_handle;
+using std::coroutine_traits; // expected-note{{declared here}}
+} // namespace std::experimental
+
+struct my_awaitable {
+  bool await_ready() noexcept;
+  void await_suspend(std::coroutine_handle<> coro) noexcept;
+  void await_resume() noexcept;
+};
+
+struct promise_void {
+  void get_return_object();
+  my_awaitable initial_suspend();
+  my_awaitable final_suspend() noexcept;
+  void return_void();
+  void unhandled_exception();
+};
+
+template <>
+struct std::coroutine_traits<void> { using promise_type = promise_void; };
+
+void test() {
+  co_return;
+  // expected-warning@-1{{support for std::experimental::coroutine_traits will be removed}}
+}

clang/test/SemaCXX/coroutine-mixed4-exp-namespace.cpp

@@ -0,0 +1,32 @@
+// RUN: %clang_cc1 -verify -std=c++20 -fsyntax-only %s
+
+#include "Inputs/std-coroutine.h"
+
+namespace std::experimental {
+// expected-note@+1{{declared here}}
+template <typename T> using coroutine_traits = std::coroutine_traits<T>;
+using std::coroutine_handle;
+} // namespace std::experimental
+
+struct my_awaitable {
+  bool await_ready() noexcept;
+  void await_suspend(std::experimental::coroutine_handle<> coro) noexcept;
+  void await_resume() noexcept;
+};
+
+struct promise_void {
+  void get_return_object();
+  my_awaitable initial_suspend();
+  my_awaitable final_suspend() noexcept;
+  void return_void();
+  void unhandled_exception();
+};
+
+template <>
+struct std::coroutine_traits<void> { using promise_type = promise_void; };
+
+void test() {
+  co_return; // expected-error {{mixed use of std and std::experimental namespaces for coroutine components}}
+  // expected-warning@-1{{support for std::experimental::coroutine_traits will be removed}}
+  // expected-note@Inputs/std-coroutine.h:8 {{'coroutine_traits' declared here}}
+}