Change SmartMove no-op behavior to match SimpleMove (and ES6 spec)

test/mjsunit/array-unshift.js

 // Copyright 2010 the V8 project authors. All rights reserved.
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 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 19 matching lines @@
 (function() {
   var array = new Array(10);
   assertEquals(13, array.unshift('1st', '2ns', '3rd'));
   assertTrue(0 in array);
   assertTrue(1 in array);
   assertTrue(2 in array);
   assertFalse(3 in array);
 })();
 
 
-// Check that unshift with no args has a side-effect of
-// filling the holes with elements from the prototype
-// (if present, of course)
+// Check that unshift with no args has no side-effects.
 (function() {
   var len = 3;
   var array = new Array(len);
 
   var at0 = '@0';
   var at2 = '@2';
 
   Array.prototype[0] = at0;
   Array.prototype[2] = at2;
 
   // array owns nothing...
   assertFalse(array.hasOwnProperty(0));
   assertFalse(array.hasOwnProperty(1));
   assertFalse(array.hasOwnProperty(2));
 
   // ... but sees values from Array.prototype
   assertEquals(array[0], at0);
   assertEquals(array[1], undefined);
   assertEquals(array[2], at2);
 
   assertEquals(len, array.unshift());
 
   assertTrue(delete Array.prototype[0]);
   assertTrue(delete Array.prototype[2]);
 
-  // unshift makes array own 0 and 2...
-  assertTrue(array.hasOwnProperty(0));
+  // array still owns nothing...
+  assertFalse(array.hasOwnProperty(0));

[Michael Starzinger, 2014/10/22 08:45:50]

I am not convinced that this is actually spec-compliant behavior. The spec says
Array.prototype.unshift should always run through a moving loop, no matter how
many arguments are passed. The loop performs [[HasProperty]], [[Get]] and
[[Put]] on all present elements. The effect of that should be that "non-own data
properties" turn into "own data properties" just like this test originally
expected.

[adamk, 2014/10/22 17:15:35]

This appears to be a difference between ES5 and ES6, I should have checked both!
Here's ES6:

https://people.mozilla.org/~jorendorff/es6-draft.html#sec-array.prototype.uns...

In particular, step 7 is "If argCount > 0, then [run the loop]"

whereas, as you point out, ES5
(http://www.ecma-international.org/ecma-262/5.1/#sec-15.4.4.13) says nothing
about checking argCount.

Seems like something to ask an ES expert on, will start by adding rossberg and
arv to this review.

   assertFalse(array.hasOwnProperty(1));
-  assertTrue(array.hasOwnProperty(2));
+  assertFalse(array.hasOwnProperty(2));
 
   // ... so they are not affected be delete.
-  assertEquals(array[0], at0);
+  assertEquals(array[0], undefined);
   assertEquals(array[1], undefined);
-  assertEquals(array[2], at2);
+  assertEquals(array[2], undefined);
 })();
 
 
 // Now check the case with array of holes and some elements on prototype.
 (function() {
   var len = 9;
   var array = new Array(len);
   Array.prototype[3] = "@3";
   Array.prototype[7] = "@7";
 
@@ skipping 21 matching lines @@
   assertFalse(array.hasOwnProperty(3));
 
   assertEquals(array[7], Array.prototype[7]);
   assertFalse(array.hasOwnProperty(7));
 
   assertTrue(delete Array.prototype[3]);
   assertTrue(delete Array.prototype[5]);
   assertTrue(delete Array.prototype[7]);
 })();
 
-// Check that unshift with no args has a side-effect of
-// filling the holes with elements from the prototype
-// (if present, of course)
+// Check that unshift with no args has no side-effects.
 (function() {
   var len = 3;
   var array = new Array(len);
 
   var at0 = '@0';
   var at2 = '@2';
 
   var array_proto = [];
   array_proto[0] = at0;
   array_proto[2] = at2;
   array.__proto__ = array_proto;
 
   // array owns nothing...
   assertFalse(array.hasOwnProperty(0));
   assertFalse(array.hasOwnProperty(1));
   assertFalse(array.hasOwnProperty(2));
 
   // ... but sees values from array_proto.
   assertEquals(array[0], at0);
   assertEquals(array[1], undefined);
   assertEquals(array[2], at2);
 
   assertEquals(len, array.unshift());
 
-  // unshift makes array own 0 and 2...
-  assertTrue(array.hasOwnProperty(0));
+  // array still owns nothing.
+  assertFalse(array.hasOwnProperty(0));
   assertFalse(array.hasOwnProperty(1));
-  assertTrue(array.hasOwnProperty(2));
+  assertFalse(array.hasOwnProperty(2));
 
-  // ... so they are not affected be delete.
+  // ... but still sees values from array_proto.
   assertEquals(array[0], at0);
   assertEquals(array[1], undefined);
   assertEquals(array[2], at2);
 })();
 
 
 // Now check the case with array of holes and some elements on prototype.
 (function() {
   var len = 9;
   var array = new Array(len);
@@ skipping 60 matching lines @@
   array.unshift(1)
   assertEquals([1, 2, 3], array);
 
   array = [2];
   array.length = 2;
   array.__proto__[1] = 3;
   Object.defineProperty(array.__proto__, '1', {enumerable: false});
   array.unshift(1);
   assertEquals([1, 2, 3], array);
 })();