Async traits

Summary: This diff implements traits to aid in the development of templated functions accepting annotated functions.

Reviewed By: yfeldblum, A5he

Differential Revision: D21770066

fbshipit-source-id: 40ae340aeeb484c4f9769c13b5c9a92a97723801

folly/fibers/async/Async.h

@@ -16,6 +16,7 @@
 
 #pragma once
 
+#include <folly/Traits.h>
 #include <glog/logging.h>
 #include <utility>
 
@@ -65,6 +66,8 @@ T&& await(Async<T>&&);
 template <typename T>
 class [[nodiscard]] Async {
  public:
+  typedef T inner_type;
+
   // General use constructor
   template <typename... Us>
   /* implicit */ Async(Us && ... val) : val_(std::forward<Us>(val)...) {}
@@ -87,6 +90,8 @@ class [[nodiscard]] Async {
 template <>
 class [[nodiscard]] Async<void> {
  public:
+  typedef void inner_type;
+
   /* implicit */ Async() {}
   Async(const Async&) = delete;
   Async(Async && other) = default;
@@ -124,6 +129,24 @@ inline void init_await(Async<void>&& async) {
   await(std::move(async));
 }
 
+// is_async
+template <typename T>
+constexpr bool is_async_v = folly::detail::is_instantiation_of_v<Async, T>;
+
+// async_inner_type
+template <typename T>
+struct async_inner_type {
+  using type = T;
+};
+
+template <typename T>
+struct async_inner_type<Async<T>> {
+  using type = T;
+};
+
+template <typename T>
+using async_inner_type_t = typename async_inner_type<T>::type;
+
 } // namespace async
 } // namespace fibers
 } // namespace folly

folly/fibers/test/FibersTest.cpp

@@ -2791,3 +2791,12 @@ TEST(FiberManager, asyncTask) {
       .getVia(&evb);
 }
 #endif
+
+TEST(FiberManager, asyncTraits) {
+  static_assert(!async::is_async_v<int>);
+  static_assert(async::is_async_v<async::Async<int>>);
+  static_assert(
+      std::is_same<int, async::async_inner_type_t<async::Async<int>>>::value);
+  static_assert(
+      std::is_same<int&, async::async_inner_type_t<async::Async<int&>>>::value);
+}