Bug 152383 – [ES6][ES7] Drop Constructability of generator function

• Component: JavaScriptCore
• Status: RESOLVED FIXED

[Yusuke Suzuki]

In ES7, the generator function becomes non-constructible.
This is good for the current JSC implementation. We no longer need to consider about newTarget when creating a generator.
And this is already introduced in ECMA262 2016 draft.

https://github.com/tc39/ecma262/releases/tag/es2016-draft-20151201
https://github.com/tc39/tc39-notes/blob/master/es7/2015-07/july-28.md#67-new--generatorfunction
https://github.com/tc39/tc39-notes/blob/master/es7/2015-07/july-29.md#revisit-67-new--generatorfunction

[Yusuke Suzuki]

Working on this.

[Yusuke Suzuki]

Created attachment 267926 [details]
Patch

[Saam Barati]

Is there a strong chance this could change before ES7 is finalized?

[Yusuke Suzuki]

(In reply to comment #3)
> Is there a strong chance this could change before ES7 is finalized?

If  something happens or better semantics is invested, it could be changed in the future.
But I don't think this behavior will be reverted. Because of inconsistency of the current behavior, even if the currently proposed one won't be landed as is, the ES2015 behavior will be changed anyway. (So, anyway, I think we will be required to change the current form in the future)
Because,

1. The current form is not consistent to the class constructor behavior. We cannot define (class A { *constructor() { } }), but the generator is constructible.
2. As described in the https://github.com/tc39/tc39-notes/blob/master/es7/2015-07/july-28.md#67-new--generatorfunction, `new` on the generator does not provide any useful things. |this| becomes TDZ, so we cannot touch the constructed object. And since its body is not the part of the constructor, It will be called when calling next(). So there's no chance to initialize |this| with the constructor (like, setting some properties etc.) even if |this| in `new generatorFunction()` represents the constructed object.

[Saam Barati]

Comment on attachment 267926 [details]
Patch

Ok. SGTM

[WebKit Commit Bot]

Comment on attachment 267926 [details]
Patch

Clearing flags on attachment: 267926

Committed r194435: <http://trac.webkit.org/changeset/194435>

[WebKit Commit Bot]

All reviewed patches have been landed.  Closing bug.

[Yusuke Suzuki]

Reopening to attach new patch.

[Yusuke Suzuki]

Created attachment 267984 [details]
Patch

[Yusuke Suzuki]

Committed r194436: <http://trac.webkit.org/changeset/194436>