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+/*!
+@file
+Forward declares `boost::hana::detail::nested_to`.
+
+@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_DETAIL_NESTED_TO_FWD_HPP
+#define BOOST_HANA_DETAIL_NESTED_TO_FWD_HPP
+
+#include <boost/hana/config.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN namespace detail {
+ template <typename Algorithm>
+ struct nested_to_t {
+ template <typename X>
+ constexpr decltype(auto) operator()(X&& x) const;
+ };
+
+ //! @ingroup group-details
+ //! Provides a `.to` static constexpr function object.
+ //!
+ //! When creating a binary function object of type `Algo` whose signature
+ //! is `Object x Object -> Return`, `nested_to<Algo>` can be used as a base
+ //! class of `Algo`. Doing so will provide a static constexpr member called
+ //! `to`, which has the following signature:
+ //! @code
+ //! Object -> Object -> Return
+ //! @endcode
+ //!
+ //! Note that the function object `Algo` must be default-constructible,
+ //! since the algorithm will be called as `Algo{}(arguments...)`.
+ //!
+ //! @note
+ //! This function object is especially useful because it takes care of
+ //! avoiding ODR violations caused by the nested static constexpr member.
+ template <typename Algorithm>
+ struct nested_to { static constexpr nested_to_t<Algorithm> to{}; };
+
+ template <typename Algorithm>
+ constexpr nested_to_t<Algorithm> nested_to<Algorithm>::to;
+} BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_DETAIL_NESTED_TO_FWD_HPP