[coroutines] Fix checking for prvalue-ness of `await_suspend` return type

[EricWF]

Summary

@rsmith Does this correctly address the issues mentioned in https://reviews.llvm.org/D33625#inline-292971 ?

[EricWF]

LGTM.

include/clang/Basic/DiagnosticSemaKinds.td

 def err_seh_in_a_coroutine_with_cxx_exceptions : Error<
   "cannot use SEH '__try' in a coroutine when C++ exceptions are enabled">;
 def err_coroutine_promise_new_requires_nothrow : Error<
   "%0 is required to have a non-throwing noexcept specification when the promise "
    "type declares 'get_return_object_on_allocation_failure()'">;
 def note_coroutine_promise_call_implicitly_required : Note<
   "call to %0 implicitly required by coroutine function here">;
 def err_await_suspend_invalid_return_type : Error<
-  "the return type of 'await_suspend' is required to be 'void' or 'bool' (have %0)"
+  "return type of 'await_suspend' is required to be 'void' or 'bool' (have %0)"
 >;
 def note_await_ready_no_bool_conversion : Note<
-  "the return type of 'await_ready' is required to be contextually convertible to 'bool'"
+  "return type of 'await_ready' is required to be contextually convertible to 'bool'"
 >;
 }
 
 let CategoryName = "Documentation Issue" in {
 def warn_not_a_doxygen_trailing_member_comment : Warning<
   "not a Doxygen trailing comment">, InGroup<Documentation>, DefaultIgnore;
 } // end of documentation issue category
 

lib/Sema/SemaCoroutine.cpp

if (!AwaitReady->getType()->isDependentType()) {
     }
     Calls.Results[ACT::ACT_Ready] = Conv.get();
   }
   CallExpr *AwaitSuspend = cast<CallExpr>(Calls.Results[ACT::ACT_Suspend]);
   if (!AwaitSuspend->getType()->isDependentType()) {
     // [expr.await]p3 [...]
     //   - await-suspend is the expression e.await_suspend(h), which shall be
     //     a prvalue of type void or bool.
-    QualType RetType = AwaitSuspend->getType();
-    if (RetType != S.Context.BoolTy && RetType != S.Context.VoidTy) {
+    QualType RetType = AwaitSuspend->getCallReturnType(S.Context);
+    // non-class prvalues always have cv-unqualified types
+    QualType AdjRetType = RetType.getUnqualifiedType();
+    if (RetType->isReferenceType() ||
+        (AdjRetType != S.Context.BoolTy && AdjRetType != S.Context.VoidTy)) {
       S.Diag(AwaitSuspend->getCalleeDecl()->getLocation(),
              diag::err_await_suspend_invalid_return_type)
           << RetType;
       S.Diag(Loc, diag::note_coroutine_promise_call_implicitly_required)
           << AwaitSuspend->getDirectCallee();
       Calls.IsInvalid = true;
     }
   }

test/SemaCXX/coroutines.cpp

 // diagnostic.
 coro<bad_promise_no_return_func> no_return_value_or_return_void() {
   // expected-error@-1 {{'bad_promise_no_return_func' must declare either 'return_value' or 'return_void'}}
   co_await a;
 }
 
 struct bad_await_suspend_return {
   bool await_ready();
-  // expected-error@+1 {{the return type of 'await_suspend' is required to be 'void' or 'bool' (have 'char')}}
+  // expected-error@+1 {{return type of 'await_suspend' is required to be 'void' or 'bool' (have 'char')}}
   char await_suspend(std::experimental::coroutine_handle<>);
   void await_resume();
 };
 struct bad_await_ready_return {
-  // expected-note@+1 {{the return type of 'await_ready' is required to be contextually convertible to 'bool'}}
+  // expected-note@+1 {{return type of 'await_ready' is required to be contextually convertible to 'bool'}}
   void await_ready();
   bool await_suspend(std::experimental::coroutine_handle<>);
   void await_resume();
 };
 struct await_ready_explicit_bool {
   struct BoolT {
     explicit operator bool() const;
   };
   BoolT await_ready();
   void await_suspend(std::experimental::coroutine_handle<>);
   void await_resume();
 };
+template <class SuspendTy>
+struct await_suspend_type_test {
+  bool await_ready();
+  // expected-error@+2 {{return type of 'await_suspend' is required to be 'void' or 'bool' (have 'bool &')}}
+  // expected-error@+1 {{return type of 'await_suspend' is required to be 'void' or 'bool' (have 'bool &&')}}
+  SuspendTy await_suspend(std::experimental::coroutine_handle<>);
+  void await_resume();
+};
 void test_bad_suspend() {
   {
     // FIXME: The actual error emitted here is terrible, and no number of notes can save it.
     bad_await_ready_return a;
     // expected-error@+1 {{value of type 'void' is not contextually convertible to 'bool'}}
     co_await a; // expected-note {{call to 'await_ready' implicitly required by coroutine function here}}
   }
   {
     bad_await_suspend_return b;
     co_await b; // expected-note {{call to 'await_suspend' implicitly required by coroutine function here}}
   }
   {
     await_ready_explicit_bool c;
     co_await c; // OK
   }
+  {
+    await_suspend_type_test<bool &&> a;
+    await_suspend_type_test<bool &> b;
+    await_suspend_type_test<const void> c;
+    await_suspend_type_test<const volatile bool> d;
+    co_await a; // expected-note {{call to 'await_suspend' implicitly required by coroutine function here}}
+    co_await b; // expected-note {{call to 'await_suspend' implicitly required by coroutine function here}}
+    co_await c; // OK
+    co_await d; // OK
+  }
 }