Merge 2019-07 LWG Motion 29

P1474R1 Helpful pointers for ContiguousIterator

Fixes #3033.

Author: zygoloid

source/iterators.tex

@@ -1820,17 +1820,24 @@
     RandomAccessIterator<I> &&
     DerivedFrom<@\placeholdernc{ITER_CONCEPT}@(I), contiguous_iterator_tag> &&
     is_lvalue_reference_v<iter_reference_t<I>> &&
-    Same<iter_value_t<I>, remove_cvref_t<iter_reference_t<I>>>;
+    Same<iter_value_t<I>, remove_cvref_t<iter_reference_t<I>>> &&
+    requires(const I& i) {
+      { to_address(i) } -> Same<add_pointer_t<iter_reference_t<I>>>;
+    };
 \end{codeblock}
 
 \pnum
-Let \tcode{a} and \tcode{b} be dereferenceable iterators of type \tcode{I}
-such that \tcode{b} is reachable from \tcode{a},
+Let \tcode{a} and \tcode{b} be dereferenceable iterators and
+\tcode{c} be a non-dereferenceable iterator of type \tcode{I}
+such that \tcode{b} is reachable from \tcode{a} and
+\tcode{c} is reachable from \tcode{b},
 and let \tcode{D} be \tcode{iter_difference_t<I>}.
 The type \tcode{I} models \libconcept{ContiguousIterator} only if
-\tcode{addressof(*(a + D(b - a)))}
-is equal to
-\tcode{addressof(*a) + D(b - a)}.
+\begin{itemize}
+\item \tcode{to_address(a) == addressof(*a)},
+\item \tcode{to_address(b) == to_address(a) + D(b - a)}, and
+\item \tcode{to_address(c) == to_address(a) + D(c - a)}.
+\end{itemize}
 
 \rSec2[iterator.cpp17]{\Cpp{}17 iterator requirements}
 

source/ranges.tex

@@ -661,11 +661,7 @@
 \item
   Otherwise, if \tcode{ranges::begin(E)} is a valid expression whose type models
   \tcode{ContiguousIterator},
-  \begin{codeblock}
-  ranges::begin(E) == ranges::end(E) ? nullptr : addressof(*ranges::begin(E))
-  \end{codeblock}
-
-  except that \tcode{E} is evaluated only once.
+  \tcode{to_address(ranges::begin(E))}.
 
 \item
   Otherwise, \tcode{ranges::data(E)} is ill-formed.
@@ -1127,11 +1123,11 @@
       }
 
     constexpr auto data() requires ContiguousIterator<iterator_t<D>> {
-      return ranges::empty(derived()) ? nullptr : addressof(*ranges::begin(derived()));
+      return to_address(ranges::begin(derived()));
     }
     constexpr auto data() const
       requires Range<const D> && ContiguousIterator<iterator_t<const D>> {
-        return ranges::empty(derived()) ? nullptr : addressof(*ranges::begin(derived()));
+        return to_address(ranges::begin(derived()));
       }
 
     constexpr auto size() requires ForwardRange<D> &&