Function map_error_impl

src/include/expected.hpp:2213–2219

                                              std::declval<Exp>().error())),
          detail::enable_if_t<!std::is_void<Ret>::value> * = nullptr>
constexpr auto map_error_impl(Exp &&exp, F &&f) {
  using result = expected<exp_t<Exp>, detail::decay_t<Ret>>;
  return exp.has_value()
             ? result(*std::forward<Exp>(exp))
             : result(unexpect, detail::invoke(std::forward<F>(f),
                                               std::forward<Exp>(exp).error()));
}
template <class Exp, class F,
          detail::enable_if_t<!std::is_void<exp_t<Exp>>::value> * = nullptr,
          class Ret = decltype(detail::invoke(std::declval<F>(),