有关使用MPL :: range_c与mpl :: fold使用的问题

Question

我有以下简单功能来转换MPL :: vector＆lt; ...＆gt;打包＆lt; ...＆gt;

namespace mpl = boost::mpl;

template <typename... Ts>
struct pack{};
struct A{};
struct B{};

template <typename... P>
struct mp_append_impl;

template <template <typename...> typename P, typename... Ts, typename T>
struct mp_append_impl<P<Ts...>, T>
{
    using type = P<Ts..., T>;
};

template <template <typename...> typename P, typename T>
struct mp_append_impl<P<void>, T>
{
    using type = P<T>;
};

template <typename S>
struct mpl_to_pack
{
    using SEQ = mpl::range_c<int, 0, mpl::size<S>::type::value>;
    using type =
        typename mpl::fold<SEQ, pack<void>, mp_append_impl<mpl::_1, typename mpl::at<S, mpl::_2>::type>>::type;
};

void main() {

    std::cout << std::endl
              << boost::typeindex::type_id<mpl_to_pack<mpl::vector<A, B>>::type>().pretty_name() << std::endl;
}
}  

为什么此功能输出pack＆lt; mpl _ :: void_，mpl _ :: void_＆gt;而不是pack＆lt; a，b＆gt;？