24 Iterators library [iterators]

24.3 Iterator requirements [iterator.requirements]

24.3.6 Indirect callable requirements [indirectcallable]

24.3.6.2 Indirect callable traits [indirectcallable.traits]

To implement algorithms taking projections, it is necessary to determine the projected type of an iterator's value type.

For the exposition-only alias template indirect-value-t, indirect-value-t<T> denotes

(1.1)
invoke_result_t<Proj&, indirect-value-t<I>> if T names projected<I, Proj>, and
(1.2)
iter_value_t<T>& otherwise.