Implement P0505: 'Wording for GB 50'

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@291028 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/time/time.duration/time.duration.arithmetic/op_mod=rep.pass.cpp

diff --git a/test/std/utilities/time/time.duration/time.duration.arithmetic/op_mod=rep.pass.cpp b/test/std/utilities/time/time.duration/time.duration.arithmetic/op_mod=rep.pass.cpp
index 8758e17..a4852d2 100644
--- a/test/std/utilities/time/time.duration/time.duration.arithmetic/op_mod=rep.pass.cpp
+++ b/test/std/utilities/time/time.duration/time.duration.arithmetic/op_mod=rep.pass.cpp
@@ -16,9 +16,26 @@
 #include <chrono>
 #include <cassert>
 
+#include "test_macros.h"
+
+#if TEST_STD_VER > 14
+constexpr bool test_constexpr()
+{
+    std::chrono::seconds s(11);
+    s %= 3;
+    return s.count() == 2;
+}
+#endif
+
 int main()
 {
+    {
     std::chrono::microseconds us(11);
     us %= 3;
     assert(us.count() == 2);
+    }
+    
+#if TEST_STD_VER > 14
+    static_assert(test_constexpr(), "");
+#endif
 }