23 Iterators library [iterators]

23.3 Iterator requirements [iterator.requirements]

23.3.6 Indirect callable requirements [indirectcallable] Class template projected [projected]

Class template projected is used to constrain algorithms that accept callable objects and projections ([defns.projection]).
It combines a Readable type I and a callable object type Proj into a new Readable type whose reference type is the result of applying Proj to the iter_­reference_­t of I.
namespace std {
  template<Readable I, IndirectRegularUnaryInvocable<I> Proj>
  struct projected {
    using value_type = remove_cvref_t<indirect_result_t<Proj&, I>>;
    indirect_result_t<Proj&, I> operator*() const; // not defined

  template<WeaklyIncrementable I, class Proj>
  struct incrementable_traits<projected<I, Proj>> {
    using difference_type = iter_difference_t<I>;