text: /*!
  @file
  Forward declares `boost::hana::one`.
  
  @copyright Louis Dionne 2013-2017
  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_ONE_HPP
  #define BOOST_HANA_FWD_ONE_HPP
  
  #include <boost/hana/config.hpp>
  #include <boost/hana/core/when.hpp>
  
  
  BOOST_HANA_NAMESPACE_BEGIN
      //! Identity of the `Ring` multiplication.
      //! @ingroup group-Ring
      //!
      //! @tparam R
      //! The tag (must be a model of `Ring`) of the returned identity.
      //!
      //!
      //! Example
      //! -------
      //! @include example/one.cpp
  #ifdef BOOST_HANA_DOXYGEN_INVOKED
      template <typename R>
      constexpr auto one = []() -> decltype(auto) {
          return tag-dispatched;
      };
  #else
      template <typename R, typename = void>
      struct one_impl : one_impl<R, when<true>> { };
  
      template <typename R>
      struct one_t;
  
      template <typename R>
      constexpr one_t<R> one{};
  #endif
  BOOST_HANA_NAMESPACE_END
  
  #endif // !BOOST_HANA_FWD_ONE_HPP
  