struct set_union_fn
{
    template<std::input_iterator I1, std::sentinel_for<I1> S1,
             std::input_iterator I2, std::sentinel_for<I2> S2,
             std::weakly_incrementable O, class Comp = ranges::less,
             class Proj1 = std::identity, class Proj2 = std::identity>
        requires std::mergeable<I1, I2, O, Comp, Proj1, Proj2>
    constexpr ranges::set_union_result<I1, I2, O>
        operator()(I1 first1, S1 last1, I2 first2, S2 last2,
                   O d_first, Comp comp = {},
                   Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        for (; !(first1 == last1 or first2 == last2); ++d_first)
        {
            if (std::invoke(comp, std::invoke(proj1, *first1), 
                                  std::invoke(proj2, *first2)))
            {
                *d_first = *first1;
                ++first1;
            }
            else if (std::invoke(comp, std::invoke(proj2, *first2),
                                       std::invoke(proj1, *first1)))
            {
                *d_first = *first2;
                ++first2;
            }
            else
            {
                *d_first = *first1;
                ++first1;
                ++first2;
            }
        }
        auto res1 = ranges::copy(std::move(first1), std::move(last1), std::move(d_first));
        auto res2 = ranges::copy(std::move(first2), std::move(last2), std::move(res1.out));
        return {std::move(res1.in), std::move(res2.in), std::move(res2.out)};
    }
    
    template<ranges::input_range R>
    constexpr get_end(R&& r)
    {
        return ranges::end(r);
    }
    
    template<ranges::forward_range R>
    constexpr get_end(R&& r)
    {
        return ranges::next(ranges::begin(r), ranges::end(r));
    }
    
    template<ranges::input_range R1, ranges::input_range R2,
             std::weakly_incrementable O, class Comp = ranges::less,
             class Proj1 = std::identity, class Proj2 = std::identity>
        requires std::mergeable<ranges::iterator_t<R1>, ranges::iterator_t<R2>,
                                O, Comp, Proj1, Proj2>
    constexpr ranges::set_union_result<ranges::borrowed_iterator_t<R1>,
                                       ranges::borrowed_iterator_t<R2>, O>
        operator()(R1&& r1, R2&& r2, O d_first, Comp comp = {},
                   Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        return (*this)(ranges::begin(r1), get_end(r1),
                       ranges::begin(r2), get_end(r2),
                       std::move(d_first), std::move(comp),
                       std::move(proj1), std::move(proj2));
    }
};

inline constexpr set_union_fn set_union{};