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
>
>
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>
>
>
> --
> Text by me above is hereby placed in the public domain
>
> Cheers,
> --MarkM
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