[iterator.concepts.general]

For a type I, let ITER_TRAITS(I) denote the type I if iterator_traits<I> names a specialization generated from the primary template.

Otherwise, ITER_TRAITS(I) denotes iterator_traits<I>.

(1.1)
If the qualified-id ITER_TRAITS(I)::iterator_concept is valid and names a type, then ITER_CONCEPT(I) denotes that type.
(1.2)
Otherwise, if the qualified-id ITER_TRAITS(I)::iterator_category is valid and names a type, then ITER_CONCEPT(I) denotes that type.
(1.3)
Otherwise, if iterator_traits<I> names a specialization generated from the primary template, then ITER_CONCEPT(I) denotes random_access_iterator_tag.
(1.4)
Otherwise, ITER_CONCEPT(I) does not denote a type.

[Note 1:

ITER_TRAITS enables independent syntactic determination of an iterator's category and concept.

— end note]

[Example 1:

struct I { using value_type = int; using difference_type = int; int operator*() const; I& operator++(); I operator++(int); I& operator--(); I operator--(int); bool operator==(I) const; }; iterator_traits<I>::iterator_category denotes input_iterator_tag, and ITER_CONCEPT(I) denotes random_access_iterator_tag.

— end example]

23 Iterators library [iterators]

23.3 Iterator requirements [iterator.requirements]

23.3.4 Iterator concepts [iterator.concepts]

23.3.4.1 General [iterator.concepts.general]