Function underlying

include/fmt/format.h:4007–4009

 */
template <typename Enum>
constexpr auto underlying(Enum e) noexcept -> underlying_t<Enum> {
  return static_cast<underlying_t<Enum>>(e);
}

namespace enums {
template <typename Enum, FMT_ENABLE_IF(std::is_enum<Enum>::value)>