Map::TryUpdate() must be in sync with Map::Update().

test/mjsunit/regress/regress-4121.js

Index: test/mjsunit/regress/regress-4121.js
diff --git a/test/mjsunit/regress/regress-4121.js b/test/mjsunit/regress/regress-4121.js
new file mode 100644
index 0000000000000000000000000000000000000000..3d777c26b355054d71cd0f0efea6a5e203af4b8a
--- /dev/null
+++ b/test/mjsunit/regress/regress-4121.js
@@ -0,0 +1,55 @@
+// Copyright 2015 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --allow-natives-syntax
+
+function Migrator(o) {
+  return o.foo;
+}
+function Loader(o) {
+  return o[0];
+}
+
+var first_smi_array = [1];
+var second_smi_array = [2];
+var first_object_array = ["first"];
+var second_object_array = ["string"];
+
+assertTrue(%HasFastSmiElements(first_smi_array));
+assertTrue(%HasFastSmiElements(second_smi_array));
+assertTrue(%HasFastObjectElements(first_object_array));
+assertTrue(%HasFastObjectElements(second_object_array));
+
+// Prepare identical transition chains for smi and object arrays.
+first_smi_array.foo = 0;
+second_smi_array.foo = 0;
+first_object_array.foo = 0;
+second_object_array.foo = 0;
+
+// Collect type feedback for not-yet-deprecated original object array map.
+for (var i = 0; i < 3; i++) Migrator(second_object_array);
+
+// Blaze a migration trail for smi array maps.
+// This marks the migrated smi array map as a migration target.
+first_smi_array.foo = 0.5;
+print(second_smi_array.foo);
+
+// Deprecate original object array map.
+// Use TryMigrate from deferred optimized code to migrate second object array.
+first_object_array.foo = 0.5;
+%OptimizeFunctionOnNextCall(Migrator);
+Migrator(second_object_array);
+
+// |second_object_array| now erroneously has a smi map.
+// Optimized code assuming smi elements will expose this.
+
+for (var i = 0; i < 3; i++) Loader(second_smi_array);
+%OptimizeFunctionOnNextCall(Loader);
+assertEquals("string", Loader(second_object_array));
+
+// Any of the following checks will also fail:
+assertTrue(%HasFastObjectElements(second_object_array));
+assertFalse(%HasFastSmiElements(second_object_array));
+assertTrue(%HaveSameMap(first_object_array, second_object_array));
+assertFalse(%HaveSameMap(first_smi_array, second_object_array));