[Bug c++/81975] New: Unpacking two packs via alias erroneously complains about mismatched argument packs

["barry.revzin at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81975

            Bug ID: 81975
           Summary: Unpacking two packs via alias erroneously complains
                    about mismatched argument packs
           Product: gcc
           Version: 7.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

Example slightly reduced from http://lists.isocpp.org/core/2017/08/2827.php:

template <class T, class U>
struct is_same;

struct make_list
{
    template <typename... Ts> 
    using f = void;
};

template<template <typename, typename> class F, typename... Ts> 
struct zip_with
{
#ifdef ALIAS
    template <typename C, typename... Ss> 
    using continuation_t = typename C::template f<F<Ts, Ss>...>;
#else
    template <typename C, typename... Ss> 
    struct continuation {
        using type = typename C::template f<F<Ts, Ss>...>;
    };  

    template <typename C, typename... Ss> 
    using continuation_t = typename continuation<C, Ss...>::type;
#endif

    template<typename... Ss> 
    using f = continuation_t<make_list, Ss...>;
};

using T = zip_with<is_same>::f<>;
using U = zip_with<is_same, int>::f<int>;
using V = zip_with<is_same, int, int>::f<int, int>;

Compiling with -DALIAS works fine. Compiling without it leads to errors for T
and V  (but not U!) about mismatched argument pack lengths while expanding
F<Ts,Ss>. The two formulations should be equivalent.