From §24.2.3/1
A class or pointer type X satisfies the requirements of an input iterator for the value type T if X satisfies the Iterator (24.2.2) and EqualityComparable (Table 17) requirements ...
Thus input iterators are required to be EqualityComparable.
All standard library container iterators must satisfy forward iterator requirements (§23.2.1 - Table 96). Since those requirements are a superset of input iterator requirements, it follows these iterators must satisfy the EqualityComparable concept.
Also, from §23.2.1 - Table 96, X::iterator is required to be convertible to
X::const_iterator.
Adding the two together answers your question that it is indeed required by the standard that comparing a Container::const_iterator to a Container::iterator is well-defined (as long as both are valid iterators pointing to the same container).
