super.prop assignment can silently overwrite non-writable properties

[Jason Orendorff]

On Mon, Apr 20, 2015 at 12:44 PM, Allen Wirfs-Brock
<allen at wirfs-brock.com> 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.

-j