Implement P0426: Constexpr for std::char_traits

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@291741 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/find.pass.cpp

diff --git a/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/find.pass.cpp b/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/find.pass.cpp
index a640af2..714202e 100644
--- a/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/find.pass.cpp
+++ b/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/find.pass.cpp
@@ -12,10 +12,24 @@
 // template<> struct char_traits<char>
 
 // static const char_type* find(const char_type* s, size_t n, const char_type& a);
+// constexpr in C++17
 
 #include <string>
 #include <cassert>
 
+#include "test_macros.h"
+
+#if TEST_STD_VER > 14
+constexpr bool test_constexpr()
+{
+    constexpr const char *p = "123";
+    return std::char_traits<char>::find(p, 3, '1') == p
+        && std::char_traits<char>::find(p, 3, '2') == p + 1
+        && std::char_traits<char>::find(p, 3, '3') == p + 2
+        && std::char_traits<char>::find(p, 3, '4') == nullptr;
+}
+#endif
+
 int main()
 {
     char s1[] = {1, 2, 3};
@@ -25,4 +39,8 @@
     assert(std::char_traits<char>::find(s1, 3, char(4)) == 0);
     assert(std::char_traits<char>::find(s1, 3, char(0)) == 0);
     assert(std::char_traits<char>::find(NULL, 0, char(0)) == 0);
+
+#if TEST_STD_VER > 14
+    static_assert(test_constexpr(), "" );
+#endif
 }