[Bug c++/81215] New: [7/8 Regression] deduction failure with variadic template template parameter

["jason at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81215

            Bug ID: 81215
           Summary: [7/8 Regression] deduction failure with variadic
                    template template parameter
           Product: gcc
           Version: 7.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: jason at gcc dot gnu.org
          Reporter: jason at gcc dot gnu.org
  Target Milestone: ---

From https://bugzilla.redhat.com/show_bug.cgi?id=1464612

template<typename U> struct X { };
template<typename T, typename U = void> struct set { };

template <typename V, template <typename...> class C>
void bar (const X<C<V>>&)
{
}

void
foo (X<set<int>>& x)
{
  bar (x);
}