Does the C++ standard require operator != must be provided for a given iterator type?

Question

The C++17 standard 27.2.1.8 says:

An iterator j is called reachable from an iterator i if and only if there is a finite sequence of applications of the expression ++i that makes i == j.

That is to say, any conforming iterator type must provide operator ==.

However, I find nothing about operator != is a requirement for iterator types.

Does the C++ standard require operator != must be provided for a given iterator type?

Answer 1

See C++17 [input.iterators]/2 Table 95 "Input iterator requirements".

Input iterators require that a != b is valid and behaves the same as !(a == b) if the latter is valid. Link to cppreference.com summary

Output iterators do not need to support either operation.