24 Iterators library [iterators]

24.3 Iterator requirements [iterator.requirements]

24.3.4 Iterator concepts [iterator.concepts]

24.3.4.9 Concept input_iterator [iterator.concept.input]

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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.
[Note 1: 
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>;