Improving flat containers interface.

base/functional.h

Index: base/functional.h
diff --git a/base/functional.h b/base/functional.h
new file mode 100644
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+++ b/base/functional.h
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+// Copyright 2017 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_H_
+#define BASE_FUNCTIONAL_H_
+
+#include <functional>
+#include <utility>
+
+#include "base/type_traits.h"
+
+namespace base {
+
+// std::less<> from C++14.
+// Needed for improved flat containers API.
+
+struct less {
+  template <typename T, typename U>
+  constexpr auto operator()(T&& lhs, U&& rhs) const
+      -> decltype(std::forward<T>(lhs) < std::forward<U>(rhs)) {
+    return std::forward<T>(lhs) < std::forward<U>(rhs);
+  }
+
+  using is_transparent = int;
+};
+
+}  // base
+
+#endif  // BASE_FUNCTIONAL_H_