Annex B 3: FunctionDeclaration in LabelledStatement in, e.g., IterationStatement?

[littledan]

With all of Annex B 3 in effect, it seems like the syntax permits statements like this in sloppy mode:

for (a in b) x: function y() {}

This is based on Annex B 3.2 https://tc39.github.io/ecma262/#sec-functiondeclarations-in-ifstatement-statement-clauses, which permits FunctionDeclarations as the LabelledItem in a LabelledStatement in sloppy mode. LabelledStatements are Statements, so they are permitted as the body of an IterationStatement.

What are the semantics of such a usage? The only guide I see is Annex B 3.4 https://tc39.github.io/ecma262/#sec-functiondeclarations-in-ifstatement-statement-clauses which says that FunctionDeclarations as the body of an IfStatement act like they are enclosed in a block. But I don't see any of this logic for LabelledStatements.

I see a couple possibilities:

• Annex B 3.2 could be made more strict by making a second type of LabelledStatement, which is only permitted as a StatementListItem and not as a Statement, which allows FunctionDeclaration as the LabelledItem, and the ordinary LabelledStatement could be left alone
• The spec could explicitly give Annex B 3.4-style semantics to a whole bunch more constructs.

Based on the general design of Annex B, trying to be minimal, I think we might want to go with the first option. Thoughts?

@bterlson @allenwb @jugglinmike

[allenwb]

See 13.7.1.1 and 13.6.1 which make usage of a labelled function illegal in those contexts.

Perhaps there should be a NOTE in B.3.2 that references back to those restrictions.

[littledan]

Thanks @allenwb ! That resolves it; I guess this is the same as my first option.

[syg]

All of Firefox, Chrome, Safari, and Edge currently still allow labelled function declarations under if, with, and loop statements in sloppy mode.

Do we know for sure if this is web compatible? I strongly suspect yes, but it seems like we still don't know.

Please re-close if there is an actual shipping implementation of this.

[syg]

I completely tested the wrong thing, and I am sorry for alerting people.