+/*
+ * Copyright 2017-present Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <functional>
+#include <type_traits>
+
+#include <folly/Traits.h>
+
+/**
+ * include or backport:
+ * * std::invoke
+ * * std::invoke_result
+ * * std::invoke_result_t
+ * * std::is_invocable
+ * * std::is_invocable_r
+ * * std::is_nothrow_invocable
+ * * std::is_nothrow_invocable_r
+ */
+
+#if __cpp_lib_invoke >= 201411 || _MSC_VER
+
+namespace folly {
+
+/* using override */ using std::invoke;
+
+}
+
+#else
+
+namespace folly {
+
+// mimic: std::invoke, C++17
+template <typename F, typename... Args>
+constexpr auto invoke(F&& f, Args&&... args) noexcept(
+ noexcept(std::forward<F>(f)(std::forward<Args>(args)...)))
+ -> decltype(std::forward<F>(f)(std::forward<Args>(args)...)) {
+ return std::forward<F>(f)(std::forward<Args>(args)...);
+}
+template <typename M, typename C, typename... Args>
+constexpr auto invoke(M(C::*d), Args&&... args)
+ -> decltype(std::mem_fn(d)(std::forward<Args>(args)...)) {
+ return std::mem_fn(d)(std::forward<Args>(args)...);
+}
+
+} // namespace folly
+
+#endif
+
+#if __cpp_lib_is_invocable >= 201703 || _MSC_VER
+
+namespace folly {
+
+/* using override */ using std::invoke_result;
+/* using override */ using std::invoke_result_t;
+/* using override */ using std::is_invocable;
+/* using override */ using std::is_invocable_r;
+/* using override */ using std::is_nothrow_invocable;
+/* using override */ using std::is_nothrow_invocable_r;
+
+}
+
+#else
+
+namespace folly {
+
+namespace detail {
+
+template <typename F, typename... Args>
+using invoke_result_ =
+ decltype(invoke(std::declval<F>(), std::declval<Args>()...));
+
+template <typename F, typename... Args>
+using invoke_nothrow_ = std::integral_constant<
+ bool,
+ noexcept(invoke(std::declval<F>(), std::declval<Args>()...))>;
+
+// from: http://en.cppreference.com/w/cpp/types/result_of, CC-BY-SA
+
+template <typename Void, typename F, typename... Args>
+struct invoke_result {};
+
+template <typename F, typename... Args>
+struct invoke_result<void_t<invoke_result_<F, Args...>>, F, Args...> {
+ using type = invoke_result_<F, Args...>;
+};
+
+template <typename Void, typename F, typename... Args>
+struct is_invocable : std::false_type {};
+
+template <typename F, typename... Args>
+struct is_invocable<void_t<invoke_result_<F, Args...>>, F, Args...>
+ : std::true_type {};
+
+template <typename Void, typename R, typename F, typename... Args>
+struct is_invocable_r : std::false_type {};
+
+template <typename R, typename F, typename... Args>
+struct is_invocable_r<void_t<invoke_result_<F, Args...>>, R, F, Args...>
+ : std::is_convertible<invoke_result_<F, Args...>, R> {};
+
+template <typename Void, typename F, typename... Args>
+struct is_nothrow_invocable : std::false_type {};
+
+template <typename F, typename... Args>
+struct is_nothrow_invocable<void_t<invoke_result_<F, Args...>>, F, Args...>
+ : invoke_nothrow_<F, Args...> {};
+
+template <typename Void, typename R, typename F, typename... Args>
+struct is_nothrow_invocable_r : std::false_type {};
+
+template <typename R, typename F, typename... Args>
+struct is_nothrow_invocable_r<void_t<invoke_result_<F, Args...>>, R, F, Args...>
+ : StrictConjunction<
+ std::is_convertible<invoke_result_<F, Args...>, R>,
+ invoke_nothrow_<F, Args...>> {};
+
+} // namespace detail
+
+// mimic: std::invoke_result, C++17
+template <typename F, typename... Args>
+struct invoke_result : detail::invoke_result<void, F, Args...> {};
+
+// mimic: std::invoke_result_t, C++17
+template <typename F, typename... Args>
+using invoke_result_t = typename invoke_result<F, Args...>::type;
+
+// mimic: std::is_invocable, C++17
+template <typename F, typename... Args>
+struct is_invocable : detail::is_invocable<void, F, Args...> {};
+
+// mimic: std::is_invocable_r, C++17
+template <typename R, typename F, typename... Args>
+struct is_invocable_r : detail::is_invocable_r<void, R, F, Args...> {};
+
+// mimic: std::is_nothrow_invocable, C++17
+template <typename F, typename... Args>
+struct is_nothrow_invocable : detail::is_nothrow_invocable<void, F, Args...> {};
+
+// mimic: std::is_nothrow_invocable_r, C++17
+template <typename R, typename F, typename... Args>
+struct is_nothrow_invocable_r
+ : detail::is_nothrow_invocable_r<void, R, F, Args...> {};
+
+} // namespace folly
+
+#endif