Rationale (February, 2012):

This is not a defect but a request for an extension and thus more appropriately addressed by EWG.

[Accepted at the November, 2016 meeting as part of paper P0195R2.]

It would be handy if a pack expansion could appear in a using-declaration:

    template <class... Mixins>
    class X : Mixins... {
      using Mixins::foo ...; // Currently not supported
    };