Find out a type of lambda or functor

hpp

#include <tuple>

template<typename Functor> struct functor_arg : functor_arg<decltype(&Functor::operator())> {};

template<typename Functor, typename ReturnType, typename... ArgTypes> struct functor_arg<ReturnType (Functor::*)(ArgTypes...) const> {
    template<std::size_t N> using arg_t = std::tuple_element_t<N, std::tuple<ArgTypes...>>;
};

int main() {
    auto func = [](int) {};
    typename functor_arg<decltype(func)>::arg_t<0> arg;
}