Added thirdparty: boost library

thirdparty/include/boost/hana/fwd/then.hpp

@@ -0,0 +1,52 @@
+/*!
+@file
+Forward declares `boost::hana::then`.
+
+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_FWD_THEN_HPP
+#define BOOST_HANA_FWD_THEN_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/when.hpp>
+
+
+namespace boost { namespace hana {
+    //! Sequentially compose two monadic actions, discarding any value
+    //! produced by the first but not its effects.
+    //! @ingroup group-Monad
+    //!
+    //!
+    //! @param before
+    //! The first `Monad` in the monadic composition chain. The result of
+    //! this monad is ignored, but its effects are combined with that of the
+    //! second monad.
+    //!
+    //! @param xs
+    //! The second `Monad` in the monadic composition chain.
+    //!
+    //!
+    //! Example
+    //! -------
+    //! @include example/then.cpp
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    constexpr auto then = [](auto&& before, auto&& xs) -> decltype(auto) {
+        return tag-dispatched;
+    };
+#else
+    template <typename M, typename = void>
+    struct then_impl : then_impl<M, when<true>> { };
+
+    struct then_t {
+        template <typename Before, typename Xs>
+        constexpr decltype(auto) operator()(Before&& before, Xs&& xs) const;
+    };
+
+    BOOST_HANA_INLINE_VARIABLE constexpr then_t then{};
+#endif
+}} // end namespace boost::hana
+
+#endif // !BOOST_HANA_FWD_THEN_HPP