25 Iterators library [iterators]

25.3 Iterator requirements [iterator.requirements]

25.3.7 Common algorithm requirements [alg.req]

25.3.7.8 Concept sortable [alg.req.sortable]

The sortable concept specifies the common requirements of algorithms that permute sequences into ordered sequences (e.g., sort).
template<class I, class R = ranges::less, class P = identity> concept sortable = permutable<I> && indirect_­strict_­weak_­order<R, projected<I, P>>;