In C++, I’m trying to figure out how one can deduce the size of the parameter pack for a function, and further how one can deduce the number of parameters of a particular type (e.g. ints)

template<int N, int N_ints, typename... Ts> 
void foo(Ts... ts){ 
   ...
}

So that I could call it like

foo(1,2,'3')

and that would be the same as

foo<3,2>(1,2,'3') 

I couldnt even get the basic problem to work, i.e.

template<int N, class... Ts>
std::array<int, N> foo(Ts... ts)
{

    constexpr int n = sizeof...(ts);
    std::array<int, n> out; 
    return out;
}


int main()
{
    auto a = foo(1, 2, 3, '4', 5);
    return 0;
}

fails because it can’t deduce N.