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diff --git a/boost/hana/union.hpp b/boost/hana/union.hpp
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+/*!
+@file
+Defines `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_UNION_HPP
+#define BOOST_HANA_UNION_HPP
+
+#include <boost/hana/fwd/union.hpp>
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/dispatch.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! @cond
+    template <typename Xs, typename Ys>
+    constexpr auto union_t::operator()(Xs&& xs, Ys&& ys) const {
+        using S = typename hana::tag_of<Xs>::type;
+        using Union = BOOST_HANA_DISPATCH_IF(union_impl<S>,
+            true
+        );
+
+        return Union::apply(static_cast<Xs&&>(xs), static_cast<Ys&&>(ys));
+    }
+    //! @endcond
+
+    template <typename S, bool condition>
+    struct union_impl<S, when<condition>> : default_ {
+        template <typename ...Args>
+        static constexpr auto apply(Args&& ...) = delete;
+    };
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_UNION_HPP