1107711 - What's the deal with yield expressions in formal default initializers?

Status: RESOLVED INVALID

(Core :: JavaScript Engine, defect)

Description

[Shu-yu Guo [:shu]]

Are they allowed or not in ES6? Seems like they are (see 14.4's passing of [Yield] into the FormalParameters production [1]) we don't correctly support this.

http://people.mozilla.org/~jorendorff/es6-draft.html#sec-generator-function-definitions

Comment 1

[André Bargull [:anba]]

No, YieldExpression cannot appear in default initializers. The [GeneratorParameter] production parameter handles this case. 

A single formal parameter resolves to the BindingElement production (14.1, 13.2.3)
FormalParameters -> FormalParameterList -> FormalsList -> FormalParameter -> BindingElement



Case A:
The formal parameter is not a BindingPattern, then we need to follow BindingElement -> SingleNameBinding. For generator functions the [GeneratorParameter] is present, so the only applicable production rule is:

SingleNameBinding[Yield,GeneratorParameter] :
  [+GeneratorParameter] BindingIdentifier[Yield] Initializer[In]opt

The Initializer production does not receiver the [Yield] parameter, so YieldExpression cannot appear within Initializer. 



Case B:
The formal parameter is a BindingPattern, in that case the only applicable rule in BindingElement is:

BindingElement[Yield, GeneratorParameter ] :
  [+GeneratorParameter] BindingPattern[?Yield,GeneratorParameter] Initializer[In]opt

And again there's no [Yield] parameter, so YieldExpression cannot appear with Initializer.



Well, actually there's also a Case C to check if YieldExpression can appear within computed property names in destructuring patterns. "function* g({[yield]: a}) {}"

To check that case we need to follow BindingProperty -> PropertyName (13.2.3, 12.2.5). If [GeneratorParameter] is present the only applicable rule is:

PropertyName[Yield,GeneratorParameter] :
  [+GeneratorParameter] ComputedPropertyName

Same reasoning as above, no [Yield] parameter is set for ComputedPropertyName, so YieldExpression cannot appear within ComputedPropertyName.

Comment 2

[André Bargull [:anba]]

yield expressions are not allowed in formal parameters, cf. ES2015, 14.4.1 Static Semantics: Early Errors:

>  It is a Syntax Error if FormalParameters Contains YieldExpression is true.

Comment 3

[Jeff Walden [:Waldo]]

If we ever mis-implemented this, I'd lay decent money on bug 1155472 having fixed it.