count_of<TypePack<int>> returns 0

I was using `meta17::count_of<TypePack<int>>` which returns 0 because the type `TypePack<int>` is not in the empty typepack.
However I was expecting to get the amount of types present in `TypePack<int>` which is 1.

Not getting a compiler error led me to believe that this is the correct function for that.
Maybe we should introduce a compiler error for such cases?