fast insertion sort for non-type template parameter packs in C++2a

insertion_sort.cpp

static auto constexpr less = [](auto&& l, auto&& r) { return l < r; };
static auto constexpr less_equal = [](auto&& l, auto&& r) { return l <= r; };
static auto constexpr greater = [](auto&& l, auto&& r) { return l > r; };
static auto constexpr greater_equal = [](auto&& l, auto&& r) { return l <= r; };

//

template <template <auto...> class C, auto... Vs, class Fn>
auto constexpr sort(C<Vs...> in, Fn)
{
    if constexpr (sizeof...(Vs) < 2)
    { return in; }
    else
    {
        auto constexpr impl = []
        <auto Head, auto... Tail, auto... SVs>
        (C<Head, Tail...>, auto& f, C<SVs...> = {})
        {
            auto constexpr l = C<(Fn{}(Head, SVs) ? Head : SVs)..., Head>{};
            auto constexpr r = C<Head, (Fn{}(Head, SVs) ? SVs : Head)...>{};

            auto constexpr sorted = []
            <auto... LVs, auto... RVs>
            (C<LVs...>, C<RVs...>)
            { return C<((LVs == Head) ? RVs : LVs)...>{}; }
            (l, r);

            if constexpr (sizeof...(Tail) == 0)
            { return sorted; }
            else
            { return f(C<Tail...>{}, f, sorted); }
        };

        return impl(in, impl);
    }
}