std::experimental::ranges::Mergeable - cppreference.com

template< class I1, class I2, class Out, class R = ranges::less<>,
          class P1 = ranges::identity, class P2 = ranges::identity >
concept bool Mergeable =
    InputIterator<I1> &&
    InputIterator<I2> &&
    WeaklyIncrementable<Out> &&
    IndirectlyCopyable<I1, Out> &&
    IndirectlyCopyable<I2, Out> &&
    IndirectStrictWeakOrder<R, ranges::projected<I1, P1>, ranges::projected<I2, P2>>;