std::mergeable - cppreference.com

template< class I1, class I2, class Out, class Comp = ranges::less,
          class Proj1 = std::identity, class Proj2 = 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<Comp,
                                    std::projected<I1, Proj1>,
                                    std::projected<I2, Proj2>>;