23 Iterators library [iterators]

23.3 Iterator requirements [iterator.requirements]

23.3.4 Iterator concepts [iterator.concepts] Concept WeaklyIncrementable [iterator.concept.winc]

The WeaklyIncrementable concept specifies the requirements on types that can be incremented with the pre- and post-increment operators.
The increment operations are not required to be equality-preserving, nor is the type required to be EqualityComparable.
template<class I>
  concept WeaklyIncrementable =
    Semiregular<I> &&
    requires(I i) {
      typename iter_difference_t<I>;
      requires SignedIntegral<iter_difference_t<I>>;
      { ++i } -> Same<I&>; // not required to be equality-preserving
      i++; // not required to be equality-preserving
Let i be an object of type I.
When i is in the domain of both pre- and post-increment, i is said to be incrementable.
I models WeaklyIncrementable<I> only if
For WeaklyIncrementable types, a equals b does not imply that ++a equals ++b.
(Equality does not guarantee the substitution property or referential transparency.)
Algorithms on weakly incrementable types should never attempt to pass through the same incrementable value twice.
They should be single-pass algorithms.
These algorithms can be used with istreams as the source of the input data through the istream_­iterator class template.
end note