Proposed resolution:

This wording is relative to N4849.

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

   friend constexpr range_difference_t<Base>
     operator-(const sentinel& y, const iterator<Const>& x)
       requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;
   

   -6- Effects: Equivalent to: return xy.end_ - yx.current_;

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

1. transform_view::iterator has an exposition-only member current_ ([range.transform.iterator])

2. transform_view::sentinel has an exposition-only member end_ ([range.transform.sentinel])

3. at [range.transform.sentinel]/6 we have:

friend constexpr range_difference_t<Base>
  operator-(const sentinel& y, const iterator<Const>& x)
    requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;

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

x is an iterator, so it has current_, not end_. y is a sentinel, so it has end_, not current_.