template<typename...>
struct type_list;

template<typename Ret, typename... Args>
auto function_signature(std::function<Ret (Args...)>) {
   struct result {
       using return_type = Ret;
       using arg_list = type_list<Args...>;
   };
   return result {};
}

template<class Lambda>
struct lambda_singature {
   using type = decltype(function_signature(std::function(std::declval<Lambda>())));
};

auto l = [] (short, long) { return 239; };
using l_signature = lambda_singature<decltype(l)>::type;
static_assert(std::is_same_v<l_signature::return_type, int>);
static_assert(std::is_same_v<l_signature::arg_list, type_list<short, long>>);