ceph/src/boost/boost/gil/detail/std_common_type.hpp

   1 //
   2 // Copyright Louis Dionne 2013-2017
   3 //
   4 // Distributed under the Boost Software License, Version 1.0.
   5 //(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   6 //
   7 #ifndef BOOST_GIL_DETAIL_STD_COMMON_TYPE_HPP
   8 #define BOOST_GIL_DETAIL_STD_COMMON_TYPE_HPP
   9
  10 #include <type_traits>
  11 #include <utility>
  12
  13 namespace boost { namespace gil { namespace detail {
  14
  15 // Defines a SFINAE-friendly version of `std::common_type`.
  16 //
  17 // Based on boost/hana/detail/std_common_type.hpp
  18 // Equivalent to `std::common_type`, except it is SFINAE-friendly and
  19 // does not support custom specializations.
  20
  21 template <typename T, typename U, typename = void>
  22 struct std_common_type {};
  23
  24 template <typename T, typename U>
  25 struct std_common_type
  26 <
  27     T, U,
  28     decltype((void)(true ? std::declval<T>() : std::declval<U>()))
  29 >
  30 {
  31     using type = typename std::decay
  32         <
  33             decltype(true ? std::declval<T>() : std::declval<U>())
  34         >::type;
  35 };
  36
  37 }}} // namespace boost::gil::detail
  38
  39 #endif