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[ceph.git] / ceph / src / boost / boost / gil / detail / std_common_type.hpp

//
// 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_GIL_DETAIL_STD_COMMON_TYPE_HPP
#define BOOST_GIL_DETAIL_STD_COMMON_TYPE_HPP

#include <type_traits>
#include <utility>

namespace boost { namespace gil { namespace detail {

// Defines a SFINAE-friendly version of `std::common_type`.
//
// Based on boost/hana/detail/std_common_type.hpp
// Equivalent to `std::common_type`, except it is SFINAE-friendly and
// does not support custom specializations.

template <typename T, typename U, typename = void>
struct std_common_type {};

template <typename T, typename U>
struct std_common_type
<
    T, U,
    decltype((void)(true ? std::declval<T>() : std::declval<U>()))
>
{
    using type = typename std::decay
        <
            decltype(true ? std::declval<T>() : std::declval<U>())
        >::type;
};

}}} // namespace boost::gil::detail

#endif