Deduction guide for Async

Summary:
This diff implements a deduction guide for explicitly creating and returning Async objects in lambdas and functions with deduced return types.

This saves the user from having to specify often lengthy return types when annotating such functions.

Reviewed By: A5he

Differential Revision: D21770141

fbshipit-source-id: f80898ef6eeadee7c5a7168aaae4031e224a0935

folly/fibers/async/Async.h

@@ -99,6 +99,15 @@ class [[nodiscard]] Async<void> {
   Async operator=(Async&&) = delete;
 };
 
+#if __cpp_deduction_guides >= 201703
+/**
+ * Deduction guide to make it easier to construct and return Async objects.
+ * The guide doesn't permit constructing and returning by reference.
+ */
+template <typename T>
+explicit Async(T)->Async<T>;
+#endif
+
 /**
  * Function to retrieve the result from the Async wrapper
  * A function calling await must return an Async wrapper itself

folly/fibers/test/FibersTest.cpp

@@ -2800,3 +2800,33 @@ TEST(FiberManager, asyncTraits) {
   static_assert(
       std::is_same<int&, async::async_inner_type_t<async::Async<int&>>>::value);
 }
+
+#if __cpp_deduction_guides >= 201703
+TEST(FiberManager, asyncConstructorGuides) {
+  auto getLiteral = []() { return async::Async(1); };
+  // int&& -> int
+  static_assert(
+      std::is_same<int, async::async_inner_type_t<decltype(getLiteral())>>::
+          value);
+
+  int i = 0;
+  auto tryGetRef = [&]() { return async::Async(static_cast<int&>(i)); };
+  // int& -> int
+  static_assert(
+      std::is_same<int, async::async_inner_type_t<decltype(tryGetRef())>>::
+          value);
+
+  auto tryGetConstRef = [&]() {
+    return async::Async(static_cast<const int&>(i));
+  };
+  // const int& -> int
+  static_assert(
+      std::is_same<int, async::async_inner_type_t<decltype(tryGetConstRef())>>::
+          value);
+
+  // int& explicitly constructed
+  auto getRef = [&]() { return async::Async<int&>(i); };
+  static_assert(
+      std::is_same<int&, async::async_inner_type_t<decltype(getRef())>>::value);
+}
+#endif