diff --git a/lib/Lex/PPExpressions.cpp b/lib/Lex/PPExpressions.cpp
index 6343a3c..3c0bf5c 100644
--- a/lib/Lex/PPExpressions.cpp
+++ b/lib/Lex/PPExpressions.cpp
@@ -328,9 +328,9 @@
   case tok::pipe:                 return 7;
   case tok::ampamp:               return 6;
   case tok::pipepipe:             return 5;
-  case tok::question:             return 4;
-  case tok::colon:                return 3;
-  case tok::comma:                return 2;
+  case tok::comma:                return 4;
+  case tok::question:             return 3;
+  case tok::colon:                return 2;
   case tok::r_paren:              return 0;   // Lowest priority, end of expr.
   case tok::eom:                  return 0;   // Lowest priority, end of macro.
   }
@@ -543,7 +543,10 @@
       Res.setIsUnsigned(false);  // C99 6.5.14p3, result is always int (signed)
       break;
     case tok::comma:
-      PP.Diag(OpToken, diag::ext_pp_comma_expr);
+      // Comma is invalid in pp expressions in c89/c++ mode, but is valid in C99
+      // if not being evaluated.
+      if (!PP.getLangOptions().C99 || ValueLive)
+        PP.Diag(OpToken, diag::ext_pp_comma_expr);
       Res = RHS; // LHS = LHS,RHS -> RHS.
       break; 
     case tok::question: {
diff --git a/test/Preprocessor/expr_comma.c b/test/Preprocessor/expr_comma.c
new file mode 100644
index 0000000..5507272
--- /dev/null
+++ b/test/Preprocessor/expr_comma.c
@@ -0,0 +1,10 @@
+// Comma is not allowed in C89
+// RUN: not clang -E %s -std=c89 -pedantic-errors 
+
+// Comma is allowed if unevaluated in C99
+// RUN: clang -E %s -std=c99 -pedantic-errors 
+
+// PR2279
+
+#if 0? 1,2:3
+#endif