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+/*!
+@file
+Forward declares `boost::hana::union_`.
+
+@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_UNION_HPP
+#define BOOST_HANA_FWD_UNION_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/when.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! Returns the union of two sets.
+    //! @relates hana::set
+    //!
+    //! Given two sets `xs` and `ys`, `union_(xs, ys)` is a new set containing
+    //! all the elements of `xs` and all the elements of `ys`, without
+    //! duplicates. For any object `x`, the following holds:
+    //! @code
+    //!     x ^in^ union_(xs, ys) if and only if x ^in^ xs || x ^in^ ys
+    //! @endcode
+    //!
+    //!
+    //! @param xs, ys
+    //! Two sets to compute the union of.
+    //!
+    //!
+    //! Example
+    //! -------
+    //! @include example/union.cpp
+    //!
+    //!
+    //! Benchmarks
+    //! ----------
+    //! <div class="benchmark-chart"
+    //!      style="min-width: 310px; height: 400px; margin: 0 auto"
+    //!      data-dataset="benchmark.union.compile.json">
+    //! </div>
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    constexpr auto union_ = [](auto&& xs, auto&& ys) {
+        return tag-dispatched;
+    };
+#else
+    template <typename S, typename = void>
+    struct union_impl : union_impl<S, when<true>> { };
+
+    struct union_t {
+        template <typename Xs, typename Ys>
+        constexpr auto operator()(Xs&& xs, Ys&& ys) const;
+    };
+
+    constexpr union_t union_{};
+#endif
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FWD_UNION_HPP