Function unpack_range

include/boost/parser/detail/text/transcode_view.hpp:701–722

        template<class R>
        constexpr decltype(auto) unpack_range(R && r)
        {
            using T = detail::remove_cv_ref_t<R>;
            if constexpr (forward_range_v<T>) {
                auto unpacked =
                    boost::parser::detail::text::unpack_iterator_and_sentinel(detail::begin(r), detail::end(r));
                if constexpr (is_bounded_array_v<T>) {
                    constexpr auto n = std::extent_v<T>;
                    if (n && !r[n - 1])
                        --unpacked.last;
                    return BOOST_PARSER_DETAIL_TEXT_SUBRANGE(unpacked.first, unpacked.last);
                } else if constexpr (
                    !std::is_same_v<decltype(unpacked.first), iterator_t<R>> ||
                    !std::is_same_v<decltype(unpacked.last), sentinel_t<R>>) {
                    return unpacking_view(std::forward<R>(r));
                } else {
                    return std::forward<R>(r);
                }
            } else {
                return std::forward<R>(r);
            }
        }

        template<class R>
        using unpacked_range = decltype(detail::unpack_range(std::declval<R>()));