add support for reference types

Summary:
This diff adds support for reference types to the Async abstraction.

One example use-case is memoization where you generate the value asynchronously on cache-miss. You want may want to return a reference to avoid the expense of copying.

Some additional context is in D21438851.

Reviewed By: yfeldblum

Differential Revision: D21478238

fbshipit-source-id: 53e1b997abed2d40f0a2f9ab935925ad9bbc91ab

folly/fibers/async/Async.h

@@ -103,7 +103,7 @@ class [[nodiscard]] Async<void> {
 template <typename T>
 T&& await(Async<T>&& async) {
   DCHECK(detail::onFiber());
-  return std::move(async.val_);
+  return static_cast<T&&>(async.val_);
 }
 
 inline void await(Async<void>&&) {

folly/fibers/test/FibersTest.cpp

@@ -2618,6 +2618,20 @@ async::Async<folly::Optional<std::string>> getOptionalAsyncString() {
 async::Async<std::tuple<int, float, std::string>> getTuple() {
   return {0, 0.0, "0"};
 }
+
+struct NonCopyableNonMoveable {
+  NonCopyableNonMoveable(const NonCopyableNonMoveable&) = delete;
+  NonCopyableNonMoveable(NonCopyableNonMoveable&&) = delete;
+  NonCopyableNonMoveable& operator=(NonCopyableNonMoveable const&) = delete;
+  NonCopyableNonMoveable& operator=(NonCopyableNonMoveable&&) = delete;
+};
+
+async::Async<NonCopyableNonMoveable const&> getReference() {
+  thread_local NonCopyableNonMoveable const value{};
+
+  return value;
+}
+
 } // namespace
 
 TEST(FiberManager, asyncAwait) {
@@ -2630,6 +2644,10 @@ TEST(FiberManager, asyncAwait) {
           EXPECT_EQ(getString(), async::init_await(getAsyncString()));
           EXPECT_EQ(getString(), *async::init_await(getOptionalAsyncString()));
           async::init_await(getTuple());
+          decltype(auto) ref = async::init_await(getReference());
+          static_assert(
+              std::is_same<decltype(ref), NonCopyableNonMoveable const&>::value,
+              "");
         })
           .getVia(&evb));
 }