如何確定 C Lambda 函數的參數和傳回類型？

template <typename T>
struct function_traits : public function_traits<decltype(&T::operator())> {};

template <typename ClassType, typename ReturnType, typename... Args>
struct function_traits<ReturnType(ClassType::*)(Args...) const>
{
    enum { arity = sizeof...(Args) };
    typedef ReturnType result_type;

    template <size_t i>
    struct arg
    {
        typedef typename std::tuple_element<i, std::tuple<Args...>>::type type;
    };
};

// Example usage
int main()
{
    auto lambda = [](int i) { return long(i * 10); };

    typedef function_traits<decltype(lambda)> traits;

    static_assert(std::is_same<long, traits::result_type>::value, "err");
    static_assert(std::is_same<int, traits::arg<0>::type>::value, "err");

    return 0;
}