type_traits.h

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#ifndef A_UTIL_UTIL_BASE_TYPE_TRAITS_H
#define A_UTIL_UTIL_BASE_TYPE_TRAITS_H
 
#include <a_util/base/feature_check/library/is_enum_v.h>
 
#include <type_traits>
 
#if defined(__GNUC__) && (__GNUC__ == 5) && defined(__QNX__)
#pragma GCC diagnostic ignored                                                                     \
    "-Wattributes" // standard type attributes are ignored when used in templates
#endif
 
#if (defined(__GNUC__) && (__GNUC__ < 7)) || (defined(_MSC_VER) && (_MSC_VER < 1920))
namespace std {
template <typename...>
using void_t = void;
} // namespace std
#endif // (defined(__GNUC__) && (__GNUC__ < 7)) || (defined(_MSC_VER) && (_MSC_VER < 1920))
 
#if !HAS_IS_ENUM_V
namespace std {
template <typename T>
constexpr bool is_enum_v = is_enum<T>::value;
} // namespace std
#endif // !HAS_IS_ENUM_V
 
namespace a_util {

template <typename T, bool = std::is_enum_v<T>>
struct underlying_type_or_type {
    using type = T;
};

template <typename T>
struct underlying_type_or_type<T, true> {
    using type = std::underlying_type_t<T>;
};

template <class T, bool is_enum = std::is_enum_v<T>>
using underlying_type_or_type_t = typename underlying_type_or_type<T, is_enum>::type;

namespace detail {
 
// 'is_explicitly_convertible_to_impl' implementations are copied from:
// @author [Hunter Kohler](https://stackoverflow.com/users/8419650/hunter-kohler) here:
//         https://stackoverflow.com/a/71563534
// @copyright [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0/)
//            (See: [What is the license for the content I post?]
//            (https://stackoverflow.com/help/licensing))
template <class T, class U, class = void>
struct is_explicitly_convertible_to_impl : std::false_type {
};
 
template <class T, class U>
struct is_explicitly_convertible_to_impl<T,
                                         U,
                                         std::void_t<decltype(static_cast<U>(std::declval<T>()))>>
    : std::true_type {
};
 
} // namespace detail

template <class T, class U>
struct is_explicitly_convertible_to : detail::is_explicitly_convertible_to_impl<T, U> {
};

template <class T, class U>
constexpr bool is_explicitly_convertible_to_v = is_explicitly_convertible_to<T, U>::value;

} // namespace a_util
 
#if defined(__GNUC__) && (__GNUC__ == 5) && defined(__QNX__)
#pragma GCC diagnostic warning                                                                     \
    "-Wattributes" // standard type attributes are ignored when used in templates
#endif
 
#endif /* A_UTIL_UTIL_BASE_TYPE_TRAITS_H */