diff -r 000000000000 -r 6474c204b198 js/src/jit-test/tests/pic/watch2a.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/src/jit-test/tests/pic/watch2a.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,18 @@
+// assignments to watched properties via ++ must not be traced
+var hits = 0;
+function counter(id, oldval, newval) {
+    hits++;
+    return newval;
+}
+
+(function () {
+    var obj = {x: 0, y: 0};
+    var a = ['x', 'y'];
+    obj.watch('z', counter);
+    for (var i = 0; i < 14; i++) {
+        obj.watch(a[+(i > 8)], counter);
+        obj.y++;
+    }
+})();
+assertEq(hits, 5);
+