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

template<typename TLambda, typename TRet, typename... TArgs>
struct LambdaTypes <TRet(TLambda::*)(TArgs...) const> {
    using return_t = TRet;

    template <size_t n>
    struct arg {
        using type = std::tuple_element_t<n, std::tuple<TArgs...>>;
    };

    template<size_t n>
    using arg_t = typename arg<n>::type;
};

int main() {
    auto l = [](int x, float) -> int {return x; };

    LambdaTypes<decltype(l)> x;

    static_assert(std::is_same<decltype(x)::return_t, int>::value, "");
    static_assert(std::is_same<decltype(x)::arg_t<0>, int>::value, "");
    static_assert(std::is_same<decltype(x)::arg_t<1>, float>::value, "");

    return 0;
}