class P1 = std::identity, class P2 = std::identity >
concept mergeable =
    std::input_iterator<I1> &&
    std::input_iterator<I2> &&
    std::weakly_incrementable<Out> &&
    std::indirectly_copyable<I1, Out> &&
    std::indirectly_copyable<I2, Out> &&
    std::indirect_strict_weak_order<R,
                                    std::projected<I1, P1>,

                                   

std::projected<I2, P2>>;