Index: lib/Sema/SemaCoroutine.cpp
===================================================================
--- lib/Sema/SemaCoroutine.cpp
+++ lib/Sema/SemaCoroutine.cpp
@@ -1526,8 +1526,8 @@
   auto *FD = cast<FunctionDecl>(CurContext);
 
   auto *ScopeInfo = getCurFunction();
-  assert(ScopeInfo->CoroutineParameterMoves.empty() &&
-         "Should not build parameter moves twice");
+  if (!ScopeInfo->CoroutineParameterMoves.empty())
+    return false;
 
   for (auto *PD : FD->parameters()) {
     if (PD->getType()->isDependentType())
Index: test/SemaCXX/coroutines.cpp
===================================================================
--- test/SemaCXX/coroutines.cpp
+++ test/SemaCXX/coroutines.cpp
@@ -87,6 +87,11 @@
   co_await a;
 }
 
+int no_promise_type_2(int a) { // expected-error {{this function cannot be a coroutine: 'std::experimental::coroutine_traits<int, int>' has no member named 'promise_type'}}
+  co_await a;
+  co_await a;
+}
+
 template <>
 struct std::experimental::coroutine_traits<double, double> { typedef int promise_type; };
 double bad_promise_type(double) { // expected-error {{this function cannot be a coroutine: 'experimental::coroutine_traits<double, double>::promise_type' (aka 'int') is not a class}}