Add debug check for null pointers passed to <string_view>

llvm-svn: 371925
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 6bc1236d395187334051b99e2fcd715a54753f33
diff --git a/include/__string b/include/__string
index a88b976..b4c8815 100644
--- a/include/__string
+++ b/include/__string
@@ -351,6 +351,18 @@
 #endif
 }
 
+
+template <class _Traits>
+_LIBCPP_INLINE_VISIBILITY
+_LIBCPP_CONSTEXPR
+inline size_t __char_traits_length_checked(const typename _Traits::char_type* __s) _NOEXCEPT {
+#if _LIBCPP_DEBUG_LEVEL >= 1
+  return __s ? _Traits::length(__s) : (_VSTD::__libcpp_debug_function(_VSTD::__libcpp_debug_info(__FILE__, __LINE__, "p == nullptr", "null pointer pass to non-null argument of char_traits<...>::length")), 0);
+#else
+  return _Traits::length(__s);
+#endif
+}
+
 inline _LIBCPP_CONSTEXPR_AFTER_CXX14
 size_t
 char_traits<wchar_t>::length(const char_type* __s) _NOEXCEPT