Y Combinator in C++2a

Y.hpp

#include <utility>

template <class F>
struct Y{
  F f;
  Y(F _f) : f{_f} {}
  template<class...arg_t>
  auto operator()(arg_t&&...arg) {return f(*this,std::forward<arg_t>(arg)...);}
};

auto fact = Y{[](auto&& self, unsigned int n)->unsigned int {
                if (n<=1) return 1;
                return n*self(n-1)}};