Function or_else_impl

crates/cpp/test_headers/expected.hpp:2327–2332

                                              std::declval<Exp>().error())),
          detail::enable_if_t<!std::is_void<Ret>::value> * = nullptr>
constexpr auto or_else_impl(Exp &&exp, F &&f) {
  static_assert(detail::is_expected<Ret>::value, "F must return an expected");
  return exp.has_value() ? std::forward<Exp>(exp)
                         : detail::invoke(std::forward<F>(f),
                                          std::forward<Exp>(exp).error());
}

template <class Exp, class F,
          class Ret = decltype(detail::invoke(std::declval<F>(),