Forward iterator and bidirectional iterator place different requirements on the result of post-increment/decrement operator. The same form should be used in each case.

Merging row from:

Table 102 -- Forward iterator requirements
Table 103 -- Bidirectional iterator requirements

    r++ : convertible to const X&
    r-- : convertible to const X&
    
    *r++ : T& if X is mutable, otherwise const T&
    *r-- : convertible to T