iterator_category_t<T> is implemented as if:
template <class> struct iterator_category { };
template <class T>
struct iterator_category<T*>
: enable_if<is_object<T>::value, random_access_iterator_tag> { };
template <class T>
struct iterator_category<T const> : iterator_category<T> { };
template <class T>
requires requires { typename T::iterator_category; }
struct iterator_category<T> {
using type = see below;
};
template <class T> using iterator_category_t
= typename iterator_category<T>::type;
Users may specialize iterator_category on user-defined types.
If T::iterator_category is valid and denotes a type, then the type iterator_category<T>::type is computed as follows:
If T::iterator_category is the same as or derives from std::random_access_iterator_tag, iterator_category<T>::type is ranges::random_access_iterator_tag.
Otherwise, if T::iterator_category is the same as or derives from std::bidirectional_iterator_tag, iterator_category<T>::type is ranges::bidirectional_iterator_tag.
Otherwise, if T::iterator_category is the same as or derives from std::forward_iterator_tag, iterator_category<T>::type is ranges::forward_iterator_tag.
Otherwise, if T::iterator_category is the same as or derives from std::input_iterator_tag, iterator_category<T>::type is ranges::input_iterator_tag.
Otherwise, if T::iterator_category is the same as or derives from std::output_iterator_tag, iterator_category<T> has no nested type.
Otherwise, iterator_category<T>::type is T::iterator_category
rvalue_reference_t<T> is implemented as if:
template <dereferenceable T>
requires see below using rvalue_reference_t
= decltype(ranges::iter_move(declval<T&>()));
The expression in the requires clause is equivalent to:
requires(T& t) { { ranges::iter_move(t) } -> auto&&; }