template<unsigned n>
struct Arg {
 template<class X, class...Xs>
 constexpr auto operator()(X x, Xs...xs) {
   return Arg<n - 1>{}(xs...);
 }
};

template<>
struct Arg<0> {
 template<class X, class...Xs>
 constexpr auto operator()(X x, Xs...) {
   return x;
 }
};

template<unsigned n>

constexpr auto arg = Arg<n>{};

// arg<2>(0, 1, 2, 3, 4, 5) == 2;