detect.hpp

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// Copyright (C) 2020-2023 Jonathan Müller and lexy contributors
// SPDX-License-Identifier: BSL-1.0
 
#ifndef LEXY_DETAIL_DETECT_HPP_INCLUDED
#define LEXY_DETAIL_DETECT_HPP_INCLUDED
 
#include <lexy/_detail/config.hpp>
 
namespace lexy::_detail
{
template <typename... Args>
using void_t = void;
 
template <template <typename...> typename Op, typename Void, typename... Args>
struct _detector : std::false_type
{
    template <typename Fallback>
    using type_or = Fallback;
};
template <template <typename...> typename Op, typename... Args>
struct _detector<Op, void_t<Op<Args...>>, Args...> : std::true_type
{
    template <typename Fallback>
    using type_or = Op<Args...>;
};
 
template <template <typename...> typename Op, typename... Args>
constexpr bool is_detected = _detector<Op, void, Args...>::value;
 
template <typename Fallback, template <typename...> typename Op, typename... Args>
using detected_or = typename _detector<Op, void, Args...>::template type_or<Fallback>;
} // namespace lexy::_detail
 
#endif // LEXY_DETAIL_DETECT_HPP_INCLUDED