#include <utility>

struct swap_counter {
    unsigned int* pcnt_ = nullptr;

    friend constexpr void swap(swap_counter& lhs, swap_counter& rhs) noexcept
    {
        std::swap(lhs.pcnt_, rhs.pcnt_);
        if (lhs.pcnt_ != nullptr)
            ++*lhs.pcnt_;
        if (rhs.pcnt_ != nullptr)
            ++*lhs.pcnt_;
    }
};

static_assert([]
{
    unsigned int cnt{};
    std::pair<swap_counter, int> pr{swap_counter{&cnt}, 0};
    pr.swap(pr);
    return cnt == 2u;
}());