25 Iterators library [iterators]

25.5 Iterator adaptors [predef.iterators]

25.5.3 Constant iterators and sentinels [const.iterators]

25.5.3.4 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.