25 Iterators library [iterators]

25.3 Iterator requirements [iterator.requirements]

25.3.7 Common algorithm requirements [alg.req]

25.3.7.5 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_­binary_­predicate<R, projected<I1, P1>, projected<I2, P2>>;