On Apr 20, 2015, at 6:52 PM, Caitlin Potter wrote:
> >>If the prop property accessed by super.prop is an accessor, super.prop = x;
> >>should invoke its setter. super.prop should invoke its getter.
> >It does. This is about what happens when that property is a data property
> >doesn't exist. What happens when we do
> >[[HomeObject]].[[GetPrototypeOf]]().[[Set]]('prop', x, this)
>
> I don’t think the accessor case does work. `ownDesc` never refers to the
> property descriptor of the receiver when O[P] is a SuperReference, so if
> there’s an `this.prop` is an accessor, and `super.prop` doesn’t exist, the
> data descriptor path is taken.
`ownDexc` refers to the property descriptor of the [[Prototype]] in this case
and if "super.prop is an accessor" that will be an accessor property
descriptor. That falls throw steps 4 and 5 and eventually invokes its setter in
step 9. The Receiver (the original `this` value only is involved as the `this`
value passed in the call to the setter.
Allen
>
>> On Apr 20, 2015, at 9:37 PM, Allen Wirfs-Brock <[email protected]> wrote:
>>
>>
>> On Apr 20, 2015, at 6:21 PM, Mark Miller wrote:
>>
>>> If the prop property accessed by super.prop is an accessor, super.prop = x;
>>> should invoke its setter. super.prop should invoke its getter.
>>
>> It does. This is about what happens when that property is a data property
>> doesn't exist. What happens when we do
>> [[HomeObject]].[[GetPrototypeOf]]().[[Set]]('prop', x, this)
>>
>> Allen
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>>
>>> On Tue, Apr 21, 2015 at 4:18 AM, Allen Wirfs-Brock <[email protected]>
>>> wrote:
>>>
>>> On Apr 20, 2015, at 12:39 PM, Jason Orendorff wrote:
>>>
>>>> On Mon, Apr 20, 2015 at 12:44 PM, Allen Wirfs-Brock
>>>> <[email protected]> wrote:
>>>>>> In the spec, 9.1.9 step 4.d.i. is where `super.prop = 2` ends up, with
>>>>>> O=X.prototype.
>>>>>
>>>>> 4.d.1 doesn't set the property, it just comes up with the property
>>>>> descriptor to use, if the `Receiver` does not already have a
>>>>> corresponding own property.
>>>>>
>>>>> 5.c+5.e checks if the corresponding own property actually exists on the
>>>>> `Receiver`.
>>>>>
>>>>> If it already exits then it does a [[DefineOwnProperty]] that only
>>>>> specifies the `value` attribute. This should respect the current
>>>>> `writable` attribute of the property and hence reject the attempt to
>>>>> change the value.
>>>>
>>>> I agree with all of this, except I don't see where the attempt is
>>>> rejected. Since the property is configurable, I think
>>>> [[DefineOwnProperty]] succeeds.
>>>>
>>>> The property is still non-writable afterwards. Only the value changes.
>>>>
>>>> So this isn't breaking the object invariants: the property in question
>>>> is configurable, so it's OK (I guess) to change the value. It's just
>>>> surprising for assignment syntax to succeed in doing it.
>>>
>>> I think it's bogus and needs to be corrected. Not only does it allow (in
>>> weird cases for [[Set]] (ie, assignment) to change the value of a
>>> non-writable property. It also means there are cases where [[Set]] will
>>> convert an accessor property to a data property.
>>>
>>> In combination, I think this is a serious bug that needs to be fix in the
>>> final published ES6 spec. The fix I propose is in 9.1.9 to replace Set 5.e
>>> as follows:
>>>
>>> 5.e If existingDescriptor is not undefined, then
>>> i. If IsAccessorDescript(existingDescript), return false.
>>> ii. If existingDescriptor.[[Writable]] is false, return false.
>>> iii. Let valueDesc be the PropertyDescriptor{[[Value]]: V}.
>>> iv. Return Receiver.[[DefineOwnProperty]](P, valueDesc).
>>>
>>> Lines 5.e.i and 5.e.ii are new additions.
>>>
>>> Thoughts?
>>> Allen
>>>
>>>
>>> _______________________________________________
>>> es-discuss mailing list
>>> [email protected]
>>> https://mail.mozilla.org/listinfo/es-discuss
>>>
>>>
>>>
>>>
>>> --
>>> Text by me above is hereby placed in the public domain
>>>
>>> Cheers,
>>> --MarkM
>>
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