ceph/src/boost/libs/hana/include/boost/hana/detail/std_common_type.hpp

   1 /*!
   2 @file
   3 Defines a SFINAE-friendly version of `std::common_type`.
   4
   5 @copyright Louis Dionne 2013-2016
   6 Distributed under the Boost Software License, Version 1.0.
   7 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   8  */
   9
  10 #ifndef BOOST_HANA_DETAIL_STD_COMMON_TYPE_HPP
  11 #define BOOST_HANA_DETAIL_STD_COMMON_TYPE_HPP
  12
  13 #include <boost/hana/config.hpp>
  14 #include <boost/hana/detail/decay.hpp>
  15
  16 #include <utility>
  17
  18
  19 BOOST_HANA_NAMESPACE_BEGIN namespace detail {
  20     //! @ingroup group-details
  21     //! Equivalent to `std::common_type`, except it is SFINAE-friendly and
  22     //! does not support custom specializations.
  23     template <typename T, typename U, typename = void>
  24     struct std_common_type { };
  25
  26     template <typename T, typename U>
  27     struct std_common_type<T, U, decltype((void)(
  28         true ? std::declval<T>() : std::declval<U>()
  29     ))> {
  30         using type = typename detail::decay<
  31             decltype(true ? std::declval<T>() : std::declval<U>())
  32         >::type;
  33     };
  34 } BOOST_HANA_NAMESPACE_END
  35
  36 #endif // !BOOST_HANA_DETAIL_STD_COMMON_TYPE_HPP