[coroutines] Fix assertion in DependentCoawaitExpr when the argument is non-dependent.

[EricWF]

Summary

A co_await arg expression has a dependent type whenever the promise type is still dependent, even if the argument to co_await is not. This is because we cannot attempt the await_transform(<arg>) until after we know the promise type.

This patch fixes an assertion in the constructor of DependentCoawaitExpr that asserted that arg must also be dependent.

[GorNishanov]

LGTM!

include/clang/AST/ExprCXX.h

 public:
   DependentCoawaitExpr(SourceLocation KeywordLoc, QualType Ty, Expr *Op,
                        UnresolvedLookupExpr *OpCoawait)
       : Expr(DependentCoawaitExprClass, Ty, VK_RValue, OK_Ordinary,
              /*TypeDependent*/ true, /*ValueDependent*/ true,
              /*InstantiationDependent*/ true,
              Op->containsUnexpandedParameterPack()),
         KeywordLoc(KeywordLoc) {
-    assert(Op->isTypeDependent() && Ty->isDependentType() &&
+    // NOTE: A co_await expression is dependent on the coroutines promise
+    // type and may be dependent even when the `Op` expression is not.
+    assert(Ty->isDependentType() &&
            "wrong constructor for non-dependent co_await/co_yield expression");
     SubExprs[0] = Op;
     SubExprs[1] = OpCoawait;
   }
 
   DependentCoawaitExpr(EmptyShell Empty)
       : Expr(DependentCoawaitExprClass, Empty) {}
 

test/SemaCXX/coroutines.cpp

if (n == 0)
     co_return 3;
   if (n == 1)
     co_return {4}; // expected-warning {{braces around scalar initializer}}
   if (n == 2)
     co_return "foo"; // expected-error {{cannot initialize a parameter of type 'int' with an lvalue of type 'const char [4]'}}
   co_return 42;
 }
 
+template <class T>
+void co_await_non_dependent_arg(T) {
+  co_await a;
+}
+template void co_await_non_dependent_arg(int);
+
 void mixed_yield() {
   co_yield 0; // expected-note {{use of 'co_yield'}}
   return; // expected-error {{not allowed in coroutine}}
 }
 
 void mixed_await() {
   co_await a; // expected-note {{use of 'co_await'}}
   return; // expected-error {{not allowed in coroutine}}