23 Iterators library [iterators]

23.3 Iterator requirements [iterator.requirements]

23.3.4 Iterator concepts [iterator.concepts] Concept input_­iterator [iterator.concept.input]

The input_­iterator concept defines requirements for a type whose referenced values can be read (from the requirement for indirectly_­readable ([iterator.concept.readable])) and which can be both pre- and post-incremented.
Unlike the Cpp17InputIterator requirements ([input.iterators]), the input_­iterator concept does not need equality comparison since iterators are typically compared to sentinels.
— end note
template<class I>
  concept input_iterator =
    input_or_output_iterator<I> &&
    indirectly_readable<I> &&
    requires { typename ITER_CONCEPT(I); } &&
    derived_from<ITER_CONCEPT(I), input_iterator_tag>;