text: /*!
  @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
  