Drop unit in folly::coro::toTask

Summary:
To preserve void type in roundtrip
Doing this generically is pretty gross (you need to check if awaiter_type<decltype(co_viaIfAsync({}, SemiAwaitable))> has a member await_resume_try), while the cost of adding explicit overloads is low as there are only two types we care about, so I chose to do it that way.

Reviewed By: yfeldblum

Differential Revision: D23407270

fbshipit-source-id: cc952e2d8fa4297dbe363e22e99192fdf858134f

folly/experimental/coro/FutureUtil.h

@@ -40,6 +40,16 @@ Task<semi_await_result_t<SemiAwaitable>> toTask(
         co_return co_await a.get();
       });
 }
+inline Task<void> toTask(Future<Unit> a) {
+  return co_invoke([a = std::move(a)]() mutable -> Task<void> {
+    co_yield co_result(co_await co_awaitTry(std::move(a)));
+  });
+}
+inline Task<void> toTask(SemiFuture<Unit> a) {
+  return co_invoke([a = std::move(a)]() mutable -> Task<void> {
+    co_yield co_result(co_await co_awaitTry(std::move(a)));
+  });
+}
 
 // Converts the given SemiAwaitable to a SemiFuture (without starting it)
 template <typename SemiAwaitable>

folly/experimental/coro/test/FutureUtilTest.cpp

@@ -31,6 +31,9 @@ static folly::coro::Task<int> makeTask() {
 static folly::coro::AsyncGenerator<int> makeGen() {
   co_yield 42;
 }
+static folly::coro::Task<void> makeVoidTask() {
+  co_return;
+}
 
 TEST(FutureUtilTest, ToTask) {
   EXPECT_EQ(folly::coro::blockingWait(folly::coro::toTask(makeTask())), 42);
@@ -101,4 +104,12 @@ TEST(FutureUtilTest, ToFuture) {
   ex.drain();
   EXPECT_TRUE(fut3.isReady());
 }
+
+TEST(FutureUtilTest, VoidRoundtrip) {
+  folly::coro::Task<void> task = makeVoidTask();
+  folly::SemiFuture<folly::Unit> semi =
+      folly::coro::toSemiFuture(std::move(task));
+  task = folly::coro::toTask(std::move(semi));
+  folly::coro::blockingWait(std::move(task));
+}
 #endif