diff --git a/lib/Lex/PPExpressions.cpp b/lib/Lex/PPExpressions.cpp
index 168c9d0..86f93e4 100644
--- a/lib/Lex/PPExpressions.cpp
+++ b/lib/Lex/PPExpressions.cpp
@@ -91,7 +91,8 @@
     // into a simple 0, unless it is the C++ keyword "true", in which case it
     // turns into "1".
     if (II->getPPKeywordID() != tok::pp_defined) {
-      PP.Diag(PeekTok, diag::warn_pp_undef_identifier) << II;
+      if (ValueLive)
+        PP.Diag(PeekTok, diag::warn_pp_undef_identifier) << II;
       Result.Val = II->getTokenID() == tok::kw_true;
       Result.Val.setIsUnsigned(false);  // "0" is signed intmax_t 0.
       Result.setRange(PeekTok.getLocation());
diff --git a/test/Preprocessor/expr_liveness.c b/test/Preprocessor/expr_liveness.c
index 3f4003e..d9e5d7c 100644
--- a/test/Preprocessor/expr_liveness.c
+++ b/test/Preprocessor/expr_liveness.c
@@ -1,4 +1,4 @@
-/* RUN: clang -E %s -DNO_ERRORS &&
+/* RUN: clang -E %s -DNO_ERRORS -Werror -Wundef &&
    RUN: not clang -E %s
  */
 
@@ -44,3 +44,9 @@
 
 
 #endif
+
+// rdar://6505352
+// -Wundef should not warn about use of undefined identifier if not live.
+#if (!defined(XXX) || XXX > 42)
+#endif
+