[Bug c++/89085] New: call of overload is ambiguous with parameter packs

[lumosimann at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89085

            Bug ID: 89085
           Summary: call of overload is ambiguous with parameter packs
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lumosimann at gmail dot com
  Target Milestone: ---

The following example behaves differently for GCC and Clang and I am almost
sure that Clang is right, because the second overload is clearly more
specialized:

    template <int, typename T, typename... Args>
    int a(T const&, Args&&...) {
        return 0;
    }

    template <int, typename... Args>
    int a(int const&, Args&&...) {
        return 1;
    }

    int main() { a<1>(3, 1); }

See https://godbolt.org/z/7eRGmW

Note that the issue requires that

1. parts of the function template needs to be specialized when calling
(removing the first template parameter resolved the problem)

2. it requires a parameter pack


Just some further notes:

- When I add another Arg to the first overload (see
https://godbolt.org/z/AF7TeC), gcc compiles and chooses the first overload, and
clang rejects, and again I think that clang is right
- One way how we can do a workaround: https://godbolt.org/z/WmKdL5