24 Iterators library [iterators]

24.3 Iterator requirements [iterator.requirements]

24.3.7 Common algorithm requirements [alg.req]

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

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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>>;