[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

llvm-svn: 346764

1 files changed, 14 insertions, 3 deletions

diff --git a/libcxx/test/std/utilities/memory/pointer.traits/pointer_to.pass.cpp b/libcxx/test/std/utilities/memory/pointer.traits/pointer_to.pass.cpp
index b3f3c24020d..756f8942969 100644
--- a/libcxx/test/std/utilities/memory/pointer.traits/pointer_to.pass.cpp
+++ b/libcxx/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
 }