Replacing the wrapper type of a parameter pack

Question

The following works, but I feel like it must be possible without resorting to using decltype on the function return type:

    typedef std::size_t SizeT;

    template<SizeT... Indices> struct IndexList { };

    template<SizeT... Is>
    constexpr decltype(auto) ExtractIndices(std::integer_sequence<SizeT, Is...>)
    {
        return IndexList<Is...>{ };
    }

    template<SizeT N>
    using MakeIndexSequence = std::make_integer_sequence<SizeT, N>;

    template<SizeT N>
    using MakeIndexList = decltype(ExtractIndices(MakeIndexSequence<N>{ }));

Is there a better way to change the "wrapper type" of a parameter pack?

Answer 1

Since you're just aliasing std::index_sequence, you could just do that more explicitly:

template<SizeT... Indices> 
using IndexList = std::index_sequence<Indices...>;

template<SizeT N>
using MakeIndexList = std::make_index_sequence<N>;

Though prefer to just use what's in the standard library. Introducing your own names is confusing.