[Bug c++/126836] New: [contracts] pack index on an argument in a post condition

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126836

            Bug ID: 126836
           Summary: [contracts] pack index on an argument in a post
                    condition
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: waffl3x at gcc dot gnu.org
  Target Milestone: ---

https://godbolt.org/z/5az8o8vhx
```
template<typename... Args>
void f(Args... args)
  post(args...[1]) {}

void go()
{
  f<int, int const>(0, 0);
}
```

Does not manifest when all type arguments are const (or reference), the
following does not emit an error...
```
  f<int const, int const>(0, 0);
```
...while this does.
```
  f<int const, int const, int>(0, 0, 0);
```