24 Iterators library [iterators]

24.5 Iterator adaptors [predef.iterators]

24.5.3 Constant iterators and sentinels [const.iterators]

24.5.3.4 Member types [const.iterators.types]

basic_const_iterator<Iterator>​::​iterator_concept is defined as follows:
The member typedef-name iterator_category is defined if and only if Iterator models forward_iterator.
In that case, basic_const_iterator<Iterator>​::​iterator_category denotes the type iterator_traits<​Iterator>​::​iterator_category.