[libcxx] Implement http://wg21.link/p1006, constexpr in pointer_traits

[ldionne]

Summary

P1006 adds support for constexpr in the specialization of pointer_traits
for raw pointers. This is necessary in order to use pointer_traits in
the upcoming constexpr containers. We expect P1006 to be voted into the
working draft for C++20 at the San Diego meeting.

[mclow.lists]

Modulo nits, this looks fine to me.
Can you add a reference to www/upcoming_meeting.html, please?

libcxx/include/memory

46

I usually add // constexpr in C++20 in places like this.

libcxx/test/std/utilities/memory/pointer.traits/pointer_to.pass.cpp

15

Again, // constexpr in C++20

23–26

This should be > 17; we only test against released values.

41

Same here: > 17

[ldionne]

Add link in upcoming_meeting.html

[mclow.lists]

In D53867#1280520, @ldionne wrote:

Add link in upcoming_meeting.html

Pardon my confusing suggestion.
What I meant was "commit a link into upcoming_meeting.html", not "Update upcoming_meeting.html as part of this patch".
Since I wasn't clear, I'll do that for you.

[ldionne]

Apply Marshall's comments

libcxx/include/memory

 struct pointer_traits<T*>
 {
     typedef T* pointer;
     typedef T element_type;
     typedef ptrdiff_t difference_type;
 
     template <class U> using rebind = U*;
 
-    static pointer pointer_to(<details>) noexcept;
+    static pointer pointer_to(<details>) noexcept; // constexpr in C++20
 };
 
 template <class T> constexpr T* to_address(T* p) noexcept; // C++20
 template <class Ptr> auto to_address(const Ptr& p) noexcept; // C++20
 
 template <class Alloc>
 struct allocator_traits
 {
#ifndef _LIBCPP_CXX03_LANG
     template <class _Up> using rebind = _Up*;
 #else
     template <class _Up> struct rebind {typedef _Up* other;};
 #endif
 
 private:
     struct __nat {};
 public:
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static pointer pointer_to(typename conditional<is_void<element_type>::value,
                                       __nat, element_type>::type& __r) _NOEXCEPT
         {return _VSTD::addressof(__r);}
 };
 
 template <class _From, class _To>
 struct __rebind_pointer {
 #ifndef _LIBCPP_CXX03_LANG

libcxx/test/std/utilities/memory/pointer.traits/pointer_to.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is dual licensed under the MIT and the University of Illinois Open
 // Source Licenses. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 // <memory>
 
 // template <class T>
 // struct pointer_traits<T*>
 // {
-//     static pointer pointer_to(<details>);
+//     static pointer pointer_to(<details>); // constexpr in C++20
 //     ...
 // };
 
 #include <memory>
 #include <cassert>
+#include "test_macros.h"
 
-int main()
-{
+#if TEST_STD_VER > 17
+constexpr
+#endif
+bool check() {
     {
         int i = 0;
         static_assert((std::is_same<int *, decltype(std::pointer_traits<int*>::pointer_to(i))>::value), "");
         int* a = std::pointer_traits<int*>::pointer_to(i);
         assert(a == &i);
     }
     {
         (std::pointer_traits<void*>::element_type)0;
     }
+    return true;
+}
+
+int main() {
+    check();
+#if TEST_STD_VER > 17
+    static_assert(check(), "");
+#endif
 }