As noted in the stackoverflow comments, GCC already supports the intrinsics needed for this feature and libstdc++ defined the type traits using those intrinsics.

I would expect any proposal in this space to discuss the previous proposals, and implementation status.

I would also expect to see more discussion about std::tuple<base&...> vs std::tuple<type_identity<base>...> vs a dedicated type_list<base...> type.  The considerable compile-time overhead of std::tuple<type_identity<T>...> could be avoided by specialising std::tuple for that case, if necessary. And we could even define type_list<T...> as an alias for that specialization.