23 Iterators library [iterators]

23.3 Iterator requirements [iterator.requirements]

23.3.7 Common algorithm requirements [alg.req]

23.3.7.5 Concept indirectly_­comparable [alg.req.ind.cmp]

The indirectly_­comparable concept specifies the common requirements of algorithms that compare values from two different sequences.
template<class I1, class I2, class R, class P1 = identity,
         class P2 = identity>
  concept indirectly_comparable =
    indirect_relation<R, projected<I1, P1>, projected<I2, P2>>;