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+// Copyright 2017 The Abseil Authors.
+//
+// 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.
+//
+// absl::base_internal::Invoke(f, args...) is an implementation of
+// INVOKE(f, args...) from section [func.require] of the C++ standard.
+//
+// [func.require]
+// Define INVOKE (f, t1, t2, ..., tN) as follows:
+// 1. (t1.*f)(t2, ..., tN) when f is a pointer to a member function of a class T
+//    and t1 is an object of type T or a reference to an object of type T or a
+//    reference to an object of a type derived from T;
+// 2. ((*t1).*f)(t2, ..., tN) when f is a pointer to a member function of a
+//    class T and t1 is not one of the types described in the previous item;
+// 3. t1.*f when N == 1 and f is a pointer to member data of a class T and t1 is
+//    an object of type T or a reference to an object of type T or a reference
+//    to an object of a type derived from T;
+// 4. (*t1).*f when N == 1 and f is a pointer to member data of a class T and t1
+//    is not one of the types described in the previous item;
+// 5. f(t1, t2, ..., tN) in all other cases.
+//
+// The implementation is SFINAE-friendly: substitution failure within Invoke()
+// isn't an error.
+
+#ifndef ABSL_BASE_INTERNAL_INVOKE_H_
+#define ABSL_BASE_INTERNAL_INVOKE_H_
+
+#include <algorithm>
+#include <type_traits>
+#include <utility>
+
+// The following code is internal implementation detail.  See the comment at the
+// top of this file for the API documentation.
+
+namespace absl {
+namespace base_internal {
+
+// The five classes below each implement one of the clauses from the definition
+// of INVOKE. The inner class template Accept<F, Args...> checks whether the
+// clause is applicable; static function template Invoke(f, args...) does the
+// invocation.
+//
+// By separating the clause selection logic from invocation we make sure that
+// Invoke() does exactly what the standard says.
+
+template <typename Derived>
+struct StrippedAccept {
+  template <typename... Args>
+  struct Accept : Derived::template AcceptImpl<typename std::remove_cv<
+                      typename std::remove_reference<Args>::type>::type...> {};
+};
+
+// (t1.*f)(t2, ..., tN) when f is a pointer to a member function of a class T
+// and t1 is an object of type T or a reference to an object of type T or a
+// reference to an object of a type derived from T.
+struct MemFunAndRef : StrippedAccept<MemFunAndRef> {
+  template <typename... Args>
+  struct AcceptImpl : std::false_type {};
+
+  template <typename R, typename C, typename... Params, typename Obj,
+            typename... Args>
+  struct AcceptImpl<R (C::*)(Params...), Obj, Args...>
+      : std::is_base_of<C, Obj> {};
+
+  template <typename R, typename C, typename... Params, typename Obj,
+            typename... Args>
+  struct AcceptImpl<R (C::*)(Params...) const, Obj, Args...>
+      : std::is_base_of<C, Obj> {};
+
+  template <typename MemFun, typename Obj, typename... Args>
+  static decltype((std::declval<Obj>().*
+                   std::declval<MemFun>())(std::declval<Args>()...))
+  Invoke(MemFun&& mem_fun, Obj&& obj, Args&&... args) {
+    return (std::forward<Obj>(obj).*
+            std::forward<MemFun>(mem_fun))(std::forward<Args>(args)...);
+  }
+};
+
+// ((*t1).*f)(t2, ..., tN) when f is a pointer to a member function of a
+// class T and t1 is not one of the types described in the previous item.
+struct MemFunAndPtr : StrippedAccept<MemFunAndPtr> {
+  template <typename... Args>
+  struct AcceptImpl : std::false_type {};
+
+  template <typename R, typename C, typename... Params, typename Ptr,
+            typename... Args>
+  struct AcceptImpl<R (C::*)(Params...), Ptr, Args...>
+      : std::integral_constant<bool, !std::is_base_of<C, Ptr>::value> {};
+
+  template <typename R, typename C, typename... Params, typename Ptr,
+            typename... Args>
+  struct AcceptImpl<R (C::*)(Params...) const, Ptr, Args...>
+      : std::integral_constant<bool, !std::is_base_of<C, Ptr>::value> {};
+
+  template <typename MemFun, typename Ptr, typename... Args>
+  static decltype(((*std::declval<Ptr>()).*
+                   std::declval<MemFun>())(std::declval<Args>()...))
+  Invoke(MemFun&& mem_fun, Ptr&& ptr, Args&&... args) {
+    return ((*std::forward<Ptr>(ptr)).*
+            std::forward<MemFun>(mem_fun))(std::forward<Args>(args)...);
+  }
+};
+
+// t1.*f when N == 1 and f is a pointer to member data of a class T and t1 is
+// an object of type T or a reference to an object of type T or a reference
+// to an object of a type derived from T.
+struct DataMemAndRef : StrippedAccept<DataMemAndRef> {
+  template <typename... Args>
+  struct AcceptImpl : std::false_type {};
+
+  template <typename R, typename C, typename Obj>
+  struct AcceptImpl<R C::*, Obj> : std::is_base_of<C, Obj> {};
+
+  template <typename DataMem, typename Ref>
+  static decltype(std::declval<Ref>().*std::declval<DataMem>()) Invoke(
+      DataMem&& data_mem, Ref&& ref) {
+    return std::forward<Ref>(ref).*std::forward<DataMem>(data_mem);
+  }
+};
+
+// (*t1).*f when N == 1 and f is a pointer to member data of a class T and t1
+// is not one of the types described in the previous item.
+struct DataMemAndPtr : StrippedAccept<DataMemAndPtr> {
+  template <typename... Args>
+  struct AcceptImpl : std::false_type {};
+
+  template <typename R, typename C, typename Ptr>
+  struct AcceptImpl<R C::*, Ptr>
+      : std::integral_constant<bool, !std::is_base_of<C, Ptr>::value> {};
+
+  template <typename DataMem, typename Ptr>
+  static decltype((*std::declval<Ptr>()).*std::declval<DataMem>()) Invoke(
+      DataMem&& data_mem, Ptr&& ptr) {
+    return (*std::forward<Ptr>(ptr)).*std::forward<DataMem>(data_mem);
+  }
+};
+
+// f(t1, t2, ..., tN) in all other cases.
+struct Callable {
+  // Callable doesn't have Accept because it's the last clause that gets picked
+  // when none of the previous clauses are applicable.
+  template <typename F, typename... Args>
+  static decltype(std::declval<F>()(std::declval<Args>()...)) Invoke(
+      F&& f, Args&&... args) {
+    return std::forward<F>(f)(std::forward<Args>(args)...);
+  }
+};
+
+// Resolves to the first matching clause.
+template <typename... Args>
+struct Invoker {
+  typedef typename std::conditional<
+      MemFunAndRef::Accept<Args...>::value, MemFunAndRef,
+      typename std::conditional<
+          MemFunAndPtr::Accept<Args...>::value, MemFunAndPtr,
+          typename std::conditional<
+              DataMemAndRef::Accept<Args...>::value, DataMemAndRef,
+              typename std::conditional<DataMemAndPtr::Accept<Args...>::value,
+                                        DataMemAndPtr, Callable>::type>::type>::
+          type>::type type;
+};
+
+// The result type of Invoke<F, Args...>.
+template <typename F, typename... Args>
+using InvokeT = decltype(Invoker<F, Args...>::type::Invoke(
+    std::declval<F>(), std::declval<Args>()...));
+
+// Invoke(f, args...) is an implementation of INVOKE(f, args...) from section
+// [func.require] of the C++ standard.
+template <typename F, typename... Args>
+InvokeT<F, Args...> Invoke(F&& f, Args&&... args) {
+  return Invoker<F, Args...>::type::Invoke(std::forward<F>(f),
+                                           std::forward<Args>(args)...);
+}
+}  // namespace base_internal
+}  // namespace absl
+
+#endif  // ABSL_BASE_INTERNAL_INVOKE_H_