[range.transform.sentinel] Reinstate transform_view::sentinel::operator-

overloads, accidentally removed during application of P1614R2.

Author: zygoloid

source/ranges.tex

@@ -3468,6 +3468,30 @@
 \effects Equivalent to: \tcode{return x.current_ == y.end_;}
 \end{itemdescr}
 
+\indexlibrary{\idxcode{operator-}!\idxcode{transform_view::sentinel}}%
+\begin{itemdecl}
+friend constexpr iter_difference_t<iterator_t<Base>>
+  operator-(const iterator<Const>& x, const sentinel& y)
+    requires SizedSentinel<sentinel_t<Base>, iterator_t<Base>>;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\effects Equivalent to: \tcode{return x.current_ - y.end_;}
+\end{itemdescr}
+
+\indexlibrary{\idxcode{operator-}!\idxcode{transform_view::sentinel}}%
+\begin{itemdecl}
+friend constexpr iter_difference_t<iterator_t<Base>>
+  operator-(const sentinel& y, const iterator<Const>& x)
+    requires SizedSentinel<sentinel_t<Base>, iterator_t<Base>>;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\effects Equivalent to: \tcode{return x.end_ - y.current_;}
+\end{itemdescr}
+
 \rSec3[range.transform.adaptor]{\tcode{view::transform}}
 
 \pnum