LWG3179 subrange should always model Range

Author: Dawn Perchik

source/ranges.tex

@@ -1253,6 +1253,9 @@
 \end{itemdecl}
 
 \begin{itemdescr}
+\pnum
+\expects \range{i}{s} is a valid range.
+
 \pnum
 \effects Initializes \tcode{begin_} with \tcode{i} and \tcode{end_} with
 \tcode{s}.
@@ -1266,7 +1269,8 @@
 
 \begin{itemdescr}
 \pnum
-\expects \tcode{n == ranges::distance(i, s)}.
+\expects \range{i}{s} is a valid range, and
+\tcode{n == ranges::distance(i, s)}.
 
 \pnum
 \effects Initializes \tcode{begin_} with \tcode{i} and \tcode{end_} with