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+/*!
+@file
+Forward declares `boost::hana::plus`.
+
+@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_FWD_PLUS_HPP
+#define BOOST_HANA_FWD_PLUS_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/when.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+ //! Associative binary operation on a `Monoid`.
+ //! @ingroup group-Monoid
+ //!
+ //! @param x, y
+ //! Two objects to combine with the `Monoid`'s binary operation.
+ //!
+ //!
+ //! Cross-type version of the method
+ //! --------------------------------
+ //! The `plus` method is "overloaded" to handle distinct data types
+ //! with certain properties. Specifically, `plus` is defined for
+ //! _distinct_ data types `A` and `B` such that
+ //! 1. `A` and `B` share a common data type `C`, as determined by the
+ //! `common` metafunction
+ //! 2. `A`, `B` and `C` are all `Monoid`s when taken individually
+ //! 3. `to<C> : A -> B` and `to<C> : B -> C` are `Monoid`-embeddings, as
+ //! determined by the `is_embedding` metafunction.
+ //!
+ //! The definition of `plus` for data types satisfying the above
+ //! properties is obtained by setting
+ //! @code
+ //! plus(x, y) = plus(to<C>(x), to<C>(y))
+ //! @endcode
+ //!
+ //!
+ //! Example
+ //! -------
+ //! @include example/plus.cpp
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+ constexpr auto plus = [](auto&& x, auto&& y) -> decltype(auto) {
+ return tag-dispatched;
+ };
+#else
+ template <typename T, typename U, typename = void>
+ struct plus_impl : plus_impl<T, U, when<true>> { };
+
+ struct plus_t {
+ template <typename X, typename Y>
+ constexpr decltype(auto) operator()(X&& x, Y&& y) const;
+ };
+
+ constexpr plus_t plus{};
+#endif
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FWD_PLUS_HPP