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

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.

Reviewers: mclow.lists, EricWF

Subscribers: christof, dexonsmith, libcxx-commits

Differential Revision: https://reviews.llvm.org/D53867

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@346764 91177308-0d34-0410-b5e6-96231b3b80d8

include/memory

@@ -43,7 +43,7 @@ struct pointer_traits<T*>
 
     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
@@ -984,7 +984,7 @@ struct _LIBCPP_TEMPLATE_VIS pointer_traits<_Tp*>
 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);}

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

@@ -12,15 +12,18 @@
 // 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), "");
@@ -30,4 +33,12 @@ int main()
     {
         (std::pointer_traits<void*>::element_type)0;
     }
+    return true;
+}
+
+int main() {
+    check();
+#if TEST_STD_VER > 17
+    static_assert(check(), "");
+#endif
 }