[base] Add base::not_fn

This change adds a C++14 backport of C++17's std::not_fn to
base/functional.

Bug: None
Change-Id: Ib441a7210e9992786c2dc241438dc8ea538ef73e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2498486
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822128}

base/BUILD.gn

@@ -262,6 +262,7 @@ component("base") {
     "format_macros.h",
     "functional/identity.h",
     "functional/invoke.h",
+    "functional/not_fn.h",
     "gtest_prod_util.h",
     "guid.cc",
     "guid.h",
@@ -2743,6 +2744,7 @@ test("base_unittests") {
     "files/scoped_temp_dir_unittest.cc",
     "functional/identity_unittest.cc",
     "functional/invoke_unittest.cc",
+    "functional/not_fn_unittest.cc",
     "gmock_unittest.cc",
     "guid_unittest.cc",
     "hash/hash_unittest.cc",

base/functional/not_fn.h

+// Copyright 2020 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BASE_FUNCTIONAL_NOT_FN_H_
+#define BASE_FUNCTIONAL_NOT_FN_H_
+
+#include <type_traits>
+#include <utility>
+
+#include "base/functional/invoke.h"
+
+namespace base {
+
+namespace internal {
+
+template <typename F>
+struct NotFnImpl {
+  F f;
+
+  template <typename... Args>
+  constexpr decltype(auto) operator()(Args&&... args) & noexcept {
+    return !base::invoke(f, std::forward<Args>(args)...);
+  }
+
+  template <typename... Args>
+  constexpr decltype(auto) operator()(Args&&... args) const& noexcept {
+    return !base::invoke(f, std::forward<Args>(args)...);
+  }
+
+  template <typename... Args>
+  constexpr decltype(auto) operator()(Args&&... args) && noexcept {
+    return !base::invoke(std::move(f), std::forward<Args>(args)...);
+  }
+
+  template <typename... Args>
+  constexpr decltype(auto) operator()(Args&&... args) const&& noexcept {
+    return !base::invoke(std::move(f), std::forward<Args>(args)...);
+  }
+};
+
+}  // namespace internal
+
+// Implementation of C++17's std::not_fn.
+//
+// Reference:
+// - https://en.cppreference.com/w/cpp/utility/functional/not_fn
+// - https://wg21.link/func.not.fn
+template <typename F>
+constexpr internal::NotFnImpl<std::decay_t<F>> not_fn(F&& f) {
+  return {std::forward<F>(f)};
+}
+
+}  // namespace base
+
+#endif  // BASE_FUNCTIONAL_NOT_FN_H_

base/functional/not_fn_unittest.cc

+// Copyright 2020 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "base/functional/not_fn.h"
+
+#include <functional>
+
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace base {
+
+TEST(FunctionalTest, NotFn) {
+  constexpr auto not_less = base::not_fn(std::less<>());
+
+  using NotLessT = decltype(not_less);
+  static_assert(static_cast<const NotLessT&>(not_less)(1, 1), "");
+  static_assert(static_cast<NotLessT&>(not_less)(2, 1), "");
+  static_assert(static_cast<const NotLessT&&>(not_less)(2, 2), "");
+  static_assert(!static_cast<NotLessT&&>(not_less)(1, 2), "");
+}
+
+}  // namespace base