nlohmannjson/include/nlohmann/detail/meta/detected.hpp

#pragma once

#include <type_traits>

#include <nlohmann/detail/meta/void_t.hpp>

// http://en.cppreference.com/w/cpp/experimental/is_detected
namespace nlohmann
{
namespace detail
{
struct nonesuch
{
    nonesuch() = delete;
    ~nonesuch() = delete;
    nonesuch(nonesuch const&) = delete;
    void operator=(nonesuch const&) = delete;
};

template <class Default,
          class AlwaysVoid,
          template <class...> class Op,
          class... Args>
struct detector
{
    using value_t = std::false_type;
    using type = Default;
};

template <class Default, template <class...> class Op, class... Args>
struct detector<Default, void_t<Op<Args...>>, Op, Args...>
{
    using value_t = std::true_type;
    using type = Op<Args...>;
};

template <template <class...> class Op, class... Args>
using is_detected = typename detector<nonesuch, void, Op, Args...>::value_t;

template <template <class...> class Op, class... Args>
using detected_t = typename detector<nonesuch, void, Op, Args...>::type;

template <class Default, template <class...> class Op, class... Args>
using detected_or = detector<Default, void, Op, Args...>;

template <class Default, template <class...> class Op, class... Args>
using detected_or_t = typename detected_or<Default, Op, Args...>::type;

template <class Expected, template <class...> class Op, class... Args>
using is_detected_exact = std::is_same<Expected, detected_t<Op, Args...>>;

template <class To, template <class...> class Op, class... Args>
using is_detected_convertible =
    std::is_convertible<detected_t<Op, Args...>, To>;
}
}