[coroutines] Remove pass-through operator co_await; Replace it with the input expression

Reviewers: GorNishanov, rsmith

Reviewed By: GorNishanov

Differential Revision: https://reviews.llvm.org/D34216

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@305498 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/CGCoroutine.cpp

@@ -160,19 +160,6 @@ static LValueOrRValue emitSuspendExpression(CodeGenFunction &CGF, CGCoroData &Co
                                     bool ignoreResult, bool forLValue) {
   auto *E = S.getCommonExpr();
 
-  // FIXME: rsmith 5/22/2017. Does it still make sense for us to have a 
-  // UO_Coawait at all? As I recall, the only purpose it ever had was to
-  // represent a dependent co_await expression that couldn't yet be resolved to
-  // a CoawaitExpr. But now we have (and need!) a separate DependentCoawaitExpr
-  // node to store unqualified lookup results, it seems that the UnaryOperator
-  // portion of the representation serves no purpose (and as seen in this patch,
-  // it's getting in the way). Can we remove it?
-
-  // Skip passthrough operator co_await (present when awaiting on an LValue).
-  if (auto *UO = dyn_cast<UnaryOperator>(E))
-    if (UO->getOpcode() == UO_Coawait)
-      E = UO->getSubExpr();
-
   auto Binder =
       CodeGenFunction::OpaqueValueMappingData::bind(CGF, S.getOpaqueValue(), E);
   auto UnbindOnExit = llvm::make_scope_exit([&] { Binder.unbind(CGF); });

lib/Sema/SemaExpr.cpp

@@ -12057,11 +12057,17 @@ ExprResult Sema::CreateBuiltinUnaryOp(SourceLocation OpLoc,
     }
     break;
   case UO_Extension:
-  case UO_Coawait:
     resultType = Input.get()->getType();
     VK = Input.get()->getValueKind();
     OK = Input.get()->getObjectKind();
     break;
+  case UO_Coawait:
+    // It's unnessesary to represent the pass-through operator co_await in the
+    // AST; just return the input expression instead.
+    assert(!Input.get()->getType()->isDependentType() &&
+                   "the co_await expression must be non-dependant before "
+                   "building operator co_await");
+    return Input;
   }
   if (resultType.isNull() || Input.isInvalid())
     return ExprError();