23 Iterators library [iterators]

The template parameter Iterator shall either meet the requirements of a Cpp17BidirectionalIterator ([bidirectional.iterators]) or model bidirectional_­iterator ([iterator.concept.bidir]).

Additionally, Iterator shall either meet the requirements of a Cpp17RandomAccessIterator ([random.access.iterators]) or model random_­access_­iterator ([iterator.concept.random.access]) if the definitions of any of the members

• (2.1)
  operator+, operator-, operator+=, operator-= ([reverse.iter.nav]), or
• (2.2)
  operator[] ([reverse.iter.elem]),

or the non-member operators ([reverse.iter.cmp])

• (2.3)
  operator<, operator>, operator<=, operator>=, operator-, or operator+ ([reverse.iter.nonmember])

are instantiated ([temp.inst]).