Add CoInvokeWithoutArgs googletest helper

Summary: Add `CoInvokeWithoutArgs` helper, coroutine-aware version of `InvokeWithoutArgs`

Reviewed By: yfeldblum

Differential Revision: D27060013

fbshipit-source-id: 8024a5818fbe7759d3f9827d260c4e7370444bb0

folly/experimental/coro/GmockHelpers.h

@@ -56,10 +56,11 @@ namespace gmock_helpers {
 //   int fooCallCount = 0;
 //
 //   EXPECT_CALL(mock, foo(_))
-//     .WillRepeatedly(CoInvoke([&](int x) -> folly::coro::Task<int> {
-//                                ++fooCallCount;
-//                                co_return x + 1;
-//                              }));
+//     .WillRepeatedly(CoInvoke(
+//         [&](int x) -> folly::coro::Task<int> {
+//           ++fooCallCount;
+//           co_return x + 1;
+//         }));
 //
 template <typename F>
 auto CoInvoke(F&& f) {
@@ -68,6 +69,27 @@ auto CoInvoke(F&& f) {
   });
 }
 
+// CoInvoke variant that does not pass arguments to callback function.
+//
+// Example:
+//   using namespace ::testing
+//   using namespace folly::coro::gmock_helpers;
+//
+//   MockFoo mock;
+//   int fooCallCount = 0;
+//
+//   EXPECT_CALL(mock, foo(_))
+//     .WillRepeatedly(CoInvokeWithoutArgs(
+//         [&]() -> folly::coro::Task<int> {
+//           ++fooCallCount;
+//           co_return 42;
+//         }));
+template <typename F>
+auto CoInvokeWithoutArgs(F&& f) {
+  return ::testing::InvokeWithoutArgs(
+      [f = static_cast<F&&>(f)]() { return co_invoke(f); });
+}
+
 namespace detail {
 template <typename Fn>
 auto makeCoAction(Fn&& fn) {

folly/experimental/coro/test/GmockHelpersTest.cpp

@@ -73,6 +73,29 @@ TEST(CoroGTestHelpers, CoInvokeAvoidsDanglingReferences) {
   EXPECT_EQ(ret2, values);
 }
 
+TEST(CoroGTestHelpers, CoInvokeWithoutArgsTest) {
+  MockFoo mock;
+  int numCalls = 0;
+
+  EXPECT_CALL(mock, getString())
+      .WillRepeatedly(
+          CoInvokeWithoutArgs([&numCalls]() -> folly::coro::Task<std::string> {
+            if (numCalls++ == 0) {
+              co_return "123";
+            } else {
+              co_return "abc";
+            }
+          }));
+
+  auto ret = folly::coro::blockingWait(mock.getString());
+  EXPECT_EQ(ret, "123");
+  EXPECT_EQ(numCalls, 1);
+
+  ret = folly::coro::blockingWait(mock.getString());
+  EXPECT_EQ(ret, "abc");
+  EXPECT_EQ(numCalls, 2);
+}
+
 TEST(CoroGTestHelpers, CoReturnTest) {
   MockFoo mock;