Added thirdparty: boost library

thirdparty/include/boost/hana/detail/nested_to_fwd.hpp

@@ -0,0 +1,47 @@
+/*!
+@file
+Forward declares `boost::hana::detail::nested_to`.
+
+Copyright Louis Dionne 2013-2022
+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>
+
+
+namespace boost { namespace hana { 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;
+} }} // end namespace boost::hana
+
+#endif // !BOOST_HANA_DETAIL_NESTED_TO_FWD_HPP