LWG4170 contiguous_iterator should require to_address(I{})

Author: burblebee

source/iterators.tex

@@ -1969,6 +1969,7 @@
 \item \tcode{to_address(a) == addressof(*a)},
 \item \tcode{to_address(b) == to_address(a) + D(b - a)},
 \item \tcode{to_address(c) == to_address(a) + D(c - a)},
+\item \tcode{to_address(I\{\})} is well-defined,
 \item \tcode{ranges::iter_move(a)} has
 the same type, value category, and effects as \tcode{std::move(*a)}, and
 \item if \tcode{ranges::iter_swap(a, b)} is well-formed,