diff -r 000000000000 -r 6474c204b198 js/src/jsapi-tests/testRegExpInstanceProperties.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/src/jsapi-tests/testRegExpInstanceProperties.cpp	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,73 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: set ts=8 sts=4 et sw=4 tw=99:
+ */
+/* 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 "jscompartment.h"
+#include "jsgc.h"
+
+#include "jsapi-tests/tests.h"
+#include "vm/Shape.h"
+
+BEGIN_TEST(testRegExpInstanceProperties)
+{
+    jsval regexpProtoVal;
+    EVAL("RegExp.prototype", &regexpProtoVal);
+
+    JSObject *regexpProto = JSVAL_TO_OBJECT(regexpProtoVal);
+
+    if (!helper(regexpProto))
+        return false;
+
+    JS_GC(cx);
+
+    CHECK_EQUAL(regexpProto->compartment()->initialRegExpShape, nullptr);
+
+    jsval regexp;
+    EVAL("/foopy/", &regexp);
+    JSObject *robj = JSVAL_TO_OBJECT(regexp);
+
+    CHECK(robj->lastProperty());
+    CHECK_EQUAL(robj->compartment()->initialRegExpShape, robj->lastProperty());
+
+    return true;
+}
+
+/*
+ * Do this all in a nested function evaluation so as (hopefully) not to get
+ * screwed up by the conservative stack scanner when GCing.
+ */
+MOZ_NEVER_INLINE bool helper(JSObject *regexpProto)
+{
+    CHECK(!regexpProto->inDictionaryMode());
+
+    // Verify the compartment's cached shape is being used by RegExp.prototype.
+    const js::Shape *shape = regexpProto->lastProperty();
+    js::AutoShapeRooter root(cx, shape);
+    for (js::Shape::Range r = shape;
+         &r.front() != regexpProto->compartment()->initialRegExpShape;
+         r.popFront())
+    {
+         CHECK(!r.empty());
+    }
+
+    JS::RootedValue v(cx, INT_TO_JSVAL(17));
+    CHECK(JS_SetProperty(cx, regexpProto, "foopy", v));
+    v = INT_TO_JSVAL(42);
+    CHECK(JS_SetProperty(cx, regexpProto, "bunky", v));
+    CHECK(JS_DeleteProperty(cx, regexpProto, "foopy"));
+    CHECK(regexpProto->inDictionaryMode());
+
+    const js::Shape *shape2 = regexpProto->lastProperty();
+    js::AutoShapeRooter root2(cx, shape2);
+    js::Shape::Range r2 = shape2;
+    while (!r2.empty()) {
+        CHECK(&r2.front() != regexpProto->compartment()->initialRegExpShape);
+        r2.popFront();
+    }
+
+    return true;
+}
+END_TEST(testRegExpInstanceProperties)