[Bug c++/107111] New: GCC accepts invalid program involving function declaration with pack expansion

[jlame646 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107111

            Bug ID: 107111
           Summary: GCC accepts invalid program involving function
                    declaration with pack expansion
           Product: gcc
           Version: 12.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jlame646 at gmail dot com
  Target Milestone: ---

The following program is incorrectly accepted by gcc. Demo:
https://godbolt.org/z/rvfqzo5YW

```
template<typename T, typename... V> 
struct C 
{
    T v(V()...);;
};
int main()
{

    C<int> c; //works with gcc but rejected in clang 
    C<int, double, int, int> c2; //same here: works with gcc but rejected in
clang
}
```
This is invalid because the construct `V()...` is actually equivalent to
`V(),...`. So `V` is not expanded and this must be rejected.