Function expected_map_impl

include/experimental/expected.hpp:1114–1119

          enable_if_t<!is_void_v<Ret>> * = nullptr,
          class Result = expected<decay_t<Ret>, E>>
constexpr Result expected_map_impl(Exp &&exp, F &&f) {
  if (exp.has_value()) {
    return Result(invoke(std::forward<F>(f), *std::forward<Exp>(exp)));
  }
  return Result(unexpect, std::forward<Exp>(exp).error());
}

template <class Exp, class F, class T = typename decay_t<Exp>::value_type,
          class E = typename decay_t<Exp>::error_type,