[Bug c++/63849] New: [4.9/5.0 Regression] ICE on variadic alias template with wrappers

[reagentoo at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63849

            Bug ID: 63849
           Summary: [4.9/5.0 Regression] ICE on variadic alias template
                    with wrappers
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: reagentoo at gmail dot com

The following invalid (or perhaps invalid, I think) code snippet triggers an
ICE in GCC 4.9.2 and 5.0.0 (20141111):
================================================
template <class _T, class...>
using First = _T;            // we should not use this 
                             // alias with only 
                             // one pack parameter (?)

template <template <class...> class _Successor,
          int,
          class... _Xs>
struct Overlay
{
    using O = _Successor<_Xs...>;
};

template <class... _Pack>
struct List
{
    template <int _s>
    using O = typename Overlay<List, _s, _Pack...>::O;

    template <template <class...> class _S>
    using Pass = _S<_Pack...>;

    template <int _i>
    using At = typename O<_i>
    ::template Pass<First>;
};

template <int _i>
using At = typename List<int, char>
::template At<_i>;

template <int _i>
void func_crash(At<_i>&) {}

int main(int argc, char *argv[])
{
    char ccc;
    int iii;
    func_crash<0>(iii);
}
================================================
GCC 5.0.0 (20141111) output:
prog.cc: In instantiation of 'void func_crash(At<_i>&) [with int _i = 0; At<_i>
= int]':

prog.cc:31:6: internal compiler error: in write_name, at cp/mangle.c:801
void func_crash(At<_i>&) {}
     ^