Index: lib/AST/MicrosoftMangle.cpp
===================================================================
--- lib/AST/MicrosoftMangle.cpp
+++ lib/AST/MicrosoftMangle.cpp
@@ -1089,6 +1089,8 @@
   case OO_Array_New: Out << "?_U"; break;
   // <operator-name> ::= ?_V # delete[]
   case OO_Array_Delete: Out << "?_V"; break;
+  // <operator-name> ::= ?__L # co_await
+  case OO_Coawait: Out << "?__L"; break;
 
   case OO_Conditional: {
     DiagnosticsEngine &Diags = Context.getDiags();
@@ -1098,14 +1100,6 @@
     break;
   }
 
-  case OO_Coawait: {
-    DiagnosticsEngine &Diags = Context.getDiags();
-    unsigned DiagID = Diags.getCustomDiagID(DiagnosticsEngine::Error,
-      "cannot mangle this operator co_await yet");
-    Diags.Report(Loc, DiagID);
-    break;
-  }
-
   case OO_None:
   case NUM_OVERLOADED_OPERATORS:
     llvm_unreachable("Not an overloaded operator");
Index: test/CodeGenCoroutines/microsoft-abi-operator-coawait.cpp
===================================================================
--- /dev/null
+++ test/CodeGenCoroutines/microsoft-abi-operator-coawait.cpp
@@ -0,0 +1,25 @@
+// RUN: %clang_cc1 -triple x86_64-pc-windows-msvc18.0.0 -fcoroutines -emit-llvm %s -o - -std=c++14 -disable-llvm-passes | FileCheck %s
+struct no_suspend {
+  bool await_ready() { return true; }
+  template <typename F> void await_suspend(F) {}
+  void await_resume() {}
+};
+
+struct A {
+  no_suspend operator co_await() { return {}; }
+};
+
+struct B {};
+
+no_suspend operator co_await(B const&) { return {}; }
+
+// CHECK-LABEL: f(
+extern "C" void f() {
+  A a;
+  B b;
+  // CHECK: call void @"\01??__LA@@QEAA?AUno_suspend@@XZ"(
+  a.operator co_await();
+  // CHECK-NEXT: call i8 @"\01??__L@YA?AUno_suspend@@AEBUB@@@Z"(
+  operator co_await(b);
+}
+