Class error_result

include/boost/leaf/result.hpp:159–201

    result_discriminant what_;

    struct error_result
    {
        error_result( error_result && ) = default;
        error_result( error_result const & ) = delete;
        error_result & operator=( error_result const & ) = delete;

        result & r_;

        error_result( result & r ) noexcept:
            r_(r)
        {
        }

        template <class U>
        operator result<U>() noexcept
        {
            result_discriminant const what = r_.what_;
            switch(auto k = what.kind())
            {
                case result_discriminant::val:
                    return result<U>(error_id());
                case result_discriminant::err_id_capture_list:
#if BOOST_LEAF_CFG_CAPTURE
                    return result<U>(what.get_error_id().value(), std::move(r_.cap_));
#else
                    BOOST_LEAF_ASSERT(0); // Possible ODR violation.
#endif
                default:
                    BOOST_LEAF_ASSERT(k == result_discriminant::err_id);
                    (void) k;
                case result_discriminant::err_id_zero:
                    return result<U>(what.get_error_id());
            }
        }

        operator error_id() const noexcept
        {
            result_discriminant const what = r_.what_;
            return what.kind() == result_discriminant::val?
                error_id() :
                what.get_error_id();
        }
    };

    void destroy() const noexcept
    {