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.