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+/*!
+@file
+Defines `boost::hana::id`.
+
+@copyright Louis Dionne 2013-2016
+Distributed under the Boost Software License, Version 1.0.
+(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
+ */
+
+#ifndef BOOST_HANA_FUNCTIONAL_ID_HPP
+#define BOOST_HANA_FUNCTIONAL_ID_HPP
+
+#include <boost/hana/config.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! @ingroup group-functional
+    //! The identity function -- returns its argument unchanged.
+    //!
+    //! ### Example
+    //! @include example/functional/id.cpp
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    constexpr auto id = [](auto&& x) -> decltype(auto) {
+        return forwarded(x);
+    };
+#else
+    struct id_t {
+        template <typename T>
+        constexpr T operator()(T&& t) const {
+            return static_cast<T&&>(t);
+        }
+    };
+
+    constexpr id_t id{};
+#endif
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FUNCTIONAL_ID_HPP