Proposed resolution:

This wording is relative to N4849.

1. Modify [iterator.synopsis], header <iterator> synopsis, as indicated:

   namespace std {
     […]
     // [iterators.common], common iterators
     template<input_or_output_iterator I, sentinel_for<I> S>
       requires (!same_as<I, S> && copyable<I>)
         class common_iterator;
     […]
   }
   

2. Modify [common.iterator], class template common_iterator synopsis, as indicated:

   namespace std {
     template<input_or_output_iterator I, sentinel_for<I> S>
       requires (!same_as<I, S> && copyable<I>)
     class common_iterator {
     public:
       […]
     };
     […]
   }
   

[ 2020-02 Prioritized as IMMEDIATE Monday morning in Prague ]

We don't actually specify anywhere that non-copyable iterators do not have a common_iterator (and it would make little sense for them to as algorithms dealing with C++17 iterators are not expecting non-copyable things) As it stands common_iterator can be created from move only iterator but are then non-copyable themselves. P1862 already constrains common_view in a similar fashion