Proposed resolution:

This wording is relative to N4885.

1. Modify [range.elements.sentinel] as indicated:

       […]
       template<bool OtherConst>
         requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
       friend constexpr range_difference_t<Basemaybe-const<OtherConst, V>>
         operator-(const iterator<OtherConst>& x, const sentinel& y);
       
       template<bool OtherConst>
         requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
       friend constexpr range_difference_t<maybe-const<OtherConst, V>>
         operator-(const sentinel& x, const iterator<OtherConst>& y);
     };
   }
   

   […]

   template<bool OtherConst>
     requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
   friend constexpr range_difference_t<Basemaybe-const<OtherConst, V>>
     operator-(const iterator<OtherConst>& x, const sentinel& y);
   

   -5- Effects: Equivalent to: return x.current_ - y.end_;

[ 2021-06-07 Fixed by editorial issue #4603. Status changed: New → NAD Editorial. ]

Because the iterator type of the first operator- is const iterator<OtherConst>&, its return type should be range_difference_t<maybe-const<OtherConst, V>>.