Question regarding duplicate __proto__ properties

[Allen Wirfs-Brock]

On Nov 29, 2014, at 3:38 PM, Nicholas C. Zakas wrote:

> In Rev 28, B.3.1 it states:
> It is a Syntax Error if PropertyNameList of PropertyDefinitionList contains any duplicate entries for"__proto__" and at least two of those entries were obtained from productions of the formPropertyDefinition : PropertyName : AssignmentExpression .
> I noted that the duplicate name restriction was eliminated back in July[1] and was just wondering if __proto__ is indeed a special case?
> 
> Thanks.
> 
> [1] https://github.com/rwaldron/tc39-notes/blob/46d2396e02fd73121b5985d5a0fafbcdbf9c9072/es6/2014-07/jul-29.md#41-review-latest-draft

Yes, `__proto__ :  <expr>` in an object literal is a special form with different semantics than the other PropertyDefinition forms.  The relaxation of the duplicate entry was explicitly not applied to it.

It's worth noting that:
    __proto__:  <expr>
and 
    ["__proto__"] : <expr>
do not mean the same thing.

The first form does a [[SetPrototypeOf]] and is the form that the above early error applies to.
The second form does a [[DefineProperty]] and does not place any restrictions of duplicates.

Allen

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