The template parameter Iterator shall meet all the requirements of a Bidirectional Iterator ([bidirectional.iterators]).
Additionally,
Iterator
shall meet the requirements of a Random Access Iterator ([random.access.iterators])
if any of the members
operator+ ([reverse.iter.op+]),
operator- ([reverse.iter.op-]),
operator+= ([reverse.iter.op+=]),
operator-= ([reverse.iter.op-=]),
operator [] ([reverse.iter.opindex]),
or the global operators
operator< ([reverse.iter.op<]),
operator> ([reverse.iter.op>]),
operator <= ([reverse.iter.op<=]),
operator>= ([reverse.iter.op>=]),
operator- ([reverse.iter.opdiff])
or
operator+ ([reverse.iter.opsum])
are referenced in a way that requires instantiation ([temp.inst]).