Function and_then_impl

crates/cpp/test_headers/expected.hpp:2054–2060

          class Ret = decltype(detail::invoke(std::declval<F>(),
                                              *std::declval<Exp>()))>
constexpr auto and_then_impl(Exp &&exp, F &&f) {
  static_assert(detail::is_expected<Ret>::value, "F must return an expected");

  return exp.has_value()
             ? detail::invoke(std::forward<F>(f), *std::forward<Exp>(exp))
             : Ret(unexpect, std::forward<Exp>(exp).error());
}

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