diff -r 000000000000 -r 6474c204b198 xpcom/glue/nsArrayUtils.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/glue/nsArrayUtils.cpp	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,22 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsArrayUtils.h"
+
+//
+// do_QueryElementAt helper stuff
+//
+nsresult
+nsQueryArrayElementAt::operator()(const nsIID& aIID, void** aResult) const
+  {
+    nsresult status = mArray
+        ? mArray->QueryElementAt(mIndex, aIID, aResult)
+        : NS_ERROR_NULL_POINTER;
+
+    if (mErrorPtr)
+      *mErrorPtr = status;
+
+    return status;
+  }