Merge 2017-11 LWG Motion 12

Author: zygoloid

source/utilities.tex

@@ -6492,6 +6492,12 @@
   template <class Ptr> struct pointer_traits;
   template <class T> struct pointer_traits<T*>;
 
+  // \ref{pointer.conversion}, pointer conversion
+  template <class Ptr>
+    auto to_address(const Ptr& p) noexcept;
+  template <class T>
+    constexpr T* to_address(T* p) noexcept;
+
   // \ref{util.dynamic.safety}, pointer safety
   enum class pointer_safety { relaxed, preferred, strict };
   void declare_reachable(void* p);
@@ -6903,6 +6909,60 @@
 function. The second member function returns \tcode{addressof(r)}.
 \end{itemdescr}
 
+\rSec3[pointer.traits.optmem]{Pointer traits optional members}
+
+\pnum
+Specializations of \tcode{pointer_traits} may define the member declared
+in this subclause to customize the behavior of the standard library.
+
+\indexlibrarymember{to_address}{pointer_traits}%
+\begin{itemdecl}
+static element_type* to_address(pointer p) noexcept;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\returns
+A pointer of type \tcode{element_type*} that references
+the same location as the argument \tcode{p}.
+
+\pnum
+\begin{note}
+This function should be the inverse of \tcode{pointer_to}.
+If defined, it customizes the behavior of
+the non-member function
+\tcode{to_address}\iref{pointer.conversion}.
+\end{note}
+\end{itemdescr}
+
+\rSec2[pointer.conversion]{Pointer conversion}
+
+\indexlibrary{\idxcode{to_address}}%
+\begin{itemdecl}
+template <class Ptr> auto to_address(const Ptr& p) noexcept;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\returns
+\tcode{pointer_traits<Ptr>::to_address(p)} if that expression is well-formed
+(see \ref{pointer.traits.optmem}),
+otherwise \tcode{to_address(p.operator->())}.
+\end{itemdescr}
+
+\indexlibrary{\idxcode{to_address}}%
+\begin{itemdecl}
+template <class T> constexpr T* to_address(T* p) noexcept;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\requires \tcode{T} is not a function type. Otherwise the program is ill-formed.
+
+\pnum
+\returns \tcode{p}.
+\end{itemdescr}
+
 \rSec2[util.dynamic.safety]{Pointer safety}
 
 \pnum