Function expected_map_impl

crates/cpp/test_headers/expected.hpp:2102–2107

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

template <class Exp, class F,
          detail::enable_if_t<!std::is_void<exp_t<Exp>>::value> * = nullptr,