text: /*!
  @file
  Forward declares `boost::hana::first`.
  
  @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_FIRST_HPP
  #define BOOST_HANA_FWD_FIRST_HPP
  
  #include <boost/hana/config.hpp>
  #include <boost/hana/core/when.hpp>
  
  
  BOOST_HANA_NAMESPACE_BEGIN
      //! Returns the first element of a pair.
      //! @ingroup group-Product
      //!
      //! Note that if the `Product` actually stores the elements it contains,
      //! `hana::first` is required to return a lvalue reference, a lvalue
      //! reference to const or a rvalue reference to the first element, where
      //! the type of reference must match that of the pair passed to `first`.
      //! If the `Product` does not store the elements it contains (i.e. it
      //! generates them on demand), this requirement is dropped.
      //!
      //!
      //! Example
      //! -------
      //! @include example/first.cpp
  #ifdef BOOST_HANA_DOXYGEN_INVOKED
      constexpr auto first = [](auto&& product) -> decltype(auto) {
          return tag-dispatched;
      };
  #else
      template <typename P, typename = void>
      struct first_impl : first_impl<P, when<true>> { };
  
      struct first_t {
          template <typename Pair>
          constexpr decltype(auto) operator()(Pair&& pair) const;
      };
  
      constexpr first_t first{};
  #endif
  BOOST_HANA_NAMESPACE_END
  
  #endif // !BOOST_HANA_FWD_FIRST_HPP
  