Implement Uniform initialization constructor for Async

Summary:
Use template parameter packing in the constructor to make the implicit constructor usable for the uniform intialization of container types.

We want the return syntax to be unchanged to the extent possible when annotating functions.

Reviewed By: A5he

Differential Revision: D21290083

fbshipit-source-id: 140bbaf987cdfd673046e92dd263b5180c8e9daa

folly/fibers/async/Async.h

@@ -66,8 +66,8 @@ template <typename T>
 class [[nodiscard]] Async {
  public:
   // General use constructor
-  template <typename U>
-  /* implicit */ Async(U && val) : val_(std::forward<U>(val)) {}
+  template <typename... Us>
+  /* implicit */ Async(Us && ... val) : val_(std::forward<Us>(val)...) {}
 
   // Move constructor to allow eager-return of async without using await
   template <typename U>

folly/fibers/test/FibersTest.cpp

@@ -2615,31 +2615,21 @@ async::Async<folly::Optional<std::string>> getOptionalAsyncString() {
   // Async<folly::Optional<std::string>>
   return getAsyncString();
 }
+async::Async<std::tuple<int, float, std::string>> getTuple() {
+  return {0, 0.0, "0"};
+}
 } // namespace
 
 TEST(FiberManager, asyncAwait) {
   folly::EventBase evb;
   auto& fm = getFiberManager(evb);
 
-  EXPECT_NO_THROW(fm.addTaskFuture([&]() {
-                      EXPECT_NO_THROW(async::await(getAsyncNothing()));
-                      EXPECT_EQ(getString(), async::await(getAsyncString()));
-                      EXPECT_EQ(
-                          getString(), *async::await(getOptionalAsyncString()));
-                    })
-                      .getVia(&evb));
-}
-
-TEST(FiberManager, asyncInitAwait) {
-  folly::EventBase evb;
-  auto& fm = getFiberManager(evb);
-
   EXPECT_NO_THROW(
-      fm.addTaskFuture([&]() { async::init_await(getAsyncNothing()); })
-          .getVia(&evb));
-
-  EXPECT_EQ(
-      getString(),
-      fm.addTaskFuture([&]() { return async::init_await(getAsyncString()); })
+      fm.addTaskFuture([&]() {
+          async::init_await(getAsyncNothing());
+          EXPECT_EQ(getString(), async::init_await(getAsyncString()));
+          EXPECT_EQ(getString(), *async::init_await(getOptionalAsyncString()));
+          async::init_await(getTuple());
+        })
           .getVia(&evb));
 }