quicksort in C++ template metaprogramming

test.cpp

// -std=c++11
// licensed under MIT
// 2020 cos https://opensource.org/licenses/MIT
#include <iostream>

namespace li {
    template <int n> struct int32 {
        using type = int32;
        static constexpr int value = n;
    };

    template<int ...In> struct list {
        using type = list;
        static constexpr size_t size = sizeof...(In);
    };
    using null = list<>;

    template<int, typename> struct cons;
    template<int I, int ...In> struct cons<I, list<In...>>
        : list<I, In...> {};

    template<typename> struct first;
    template<int I, int ...In> struct first<list<I, In...>>
        : int32<I> {};

    template<typename, int> struct push;
    template<int I, int ...In> struct push<list<In...>, I>
        : list<In..., I> {};
    template<typename, typename> struct append;
    template<typename L1> struct append<L1, null>
        : L1 {};
    template<typename L1, int I, int ...In> struct append<L1, list<I, In...>>
        : append<typename push<L1, I>::type, typename list<In...>::type> {};

    #define FILTER_GEN(NAME, _OP_) \
    template<typename, int, bool> struct _##NAME##_h; \
    template<int target, int I> struct _##NAME##_h<list<I>, target, true> \
        : list<I> {}; \
    template<int target, int I> struct _##NAME##_h<list<I>, target, false> \
        : null {}; \
    template<int target, int I, int ...In> struct _##NAME##_h<list<I, In...>, target, true> \
        : cons<I, typename _##NAME##_h<typename list<In...>::type, target, (first<typename list<In...>::type>::value _OP_ target)>::type> {}; \
    template<int target, int I, int ...In> struct _##NAME##_h<list<I, In...>, target, false> \
        : _##NAME##_h<typename list<In...>::type, target, (first<typename list<In...>::type>::value _OP_ target)> {}; \
    \
    template<typename L, int target> struct NAME \
        : _##NAME##_h<L, target, (first<L>::value _OP_ target)> {}; \
    template<int target> struct NAME<null, target> \
        : null {};

    FILTER_GEN(less_eq, <=)
    FILTER_GEN(greater, >)
    #undef FILTER_GEN

    template<typename> struct qsort;
    template<> struct qsort<null> : null {};
    template<int I, int ...In> struct qsort<list<I, In...>> :
        append<typename qsort<typename less_eq<typename list<In...>::type, I>::type>::type,
            typename cons<I, typename qsort<typename greater<typename list<In...>::type, I>::type>::type>::type> {};

}

template<int I, int ...In> static void print_list(li::list<I, In...>) {
    std::cout << '(' << I;
    int _[] = { 0, ((void)(std::cout << ", " << In), 0)... };
    std::cout << ")\n";
}
static void print_list(li::null) {
    std::cout << "()\n";
}
template<int ...In> static void test() {
    using original = li::list<In...>;
    using sorted = li::qsort<original>;
    std::cout << "before: "; print_list(original());
    std::cout << "after : "; print_list(sorted());
    std::cout << '\n';
}

int main(void) {
    test<>();
    test<1>();
    test<8,1>();
    test<1,2,5,8,-3,2,100,4,9,3,-8,33,21,3,-4,-4,-4,-7,2,5,1,8,2,88,42,956,21,27,39,55,1,4,-5,-31,9>();
}

zoutput

before: ()
after : ()

before: (1)
after : (1)

before: (8, 1)
after : (1, 8)

before: (1, 2, 5, 8, -3, 2, 100, 4, 9, 3, -8, 33, 21, 3, -4, -4, -4, -7, 2, 5, 1, 8, 2, 88, 42, 956, 21, 27, 39, 55, 1, 4, -5, -31, 9)
after : (-31, -8, -7, -5, -4, -4, -4, -3, 1, 1, 1, 2, 2, 2, 2, 3, 3, 4, 4, 5, 5, 8, 8, 9, 9, 21, 21, 27, 33, 39, 42, 55, 88, 100, 956)