Proposed resolution:

Change [bidirectional.iterators]:

concept BidirectionalIterator<typename X> : ForwardIterator<X> { 
  MoveConstructible postdecrement_result; 
  requires HasDereference<postdecrement_result> 
        && Convertible<HasDereference<postdecrement_result>::result_type, const value_type&> 
        && Convertible<postdecrement_result, const X&>; 
  X& operator--(X&); 
  postdecrement_result operator--(X& r, int); {
     X tmp = r;
     --r;
     return tmp;
  }
}