Lexical Declarations cannot appear after a label

[ianwjhalliday]

ChakraCore currently allows lexical declarations (let, const, class, function*) to appear after a label. Labels can only prepend a Statement, not a Declaration, according to the ES spec.

See grammar at https://tc39.github.io/ecma262/#sec-ecmascript-language-statements-and-declarations

FunctionDeclaration gets an exception and is allowed, presumably for back-compat. GeneratorDeclaration does not get the exception, however, and so it not allow. See LabelledItem

Async functions presumably will also be disallowed by way of being a HoistableDeclaration and not excepted by LabelledItem.

E.g. each of these lines should be a syntax error but are not currently:

a: let x;
b: const y = 0;
c: class z { }
d: function* w() { }
e: async function u() { } // pending finalization of async function spec

[goyakin]

Isn't d: function w() { } actually valid since the grammar for LabelledItem allows a function declaration?

[ianwjhalliday]

Yes, good catch.

But ugh, this means it is not allowed for GeneratorDeclaration specifically. And presumably AsyncFunctionDeclaration later on.

I'll update the description

[ianwjhalliday]

This is notable: https://tc39.github.io/ecma262/#sec-labelled-function-declarations

[goyakin]

Test262 test cases for IterationStatement

[littledan]

Another case where a label is inappropriately allowed is a label immediately before the end of script, e.g., eval("x:") (which JSC, V8 and SpiderMonkey reject, but ChakraCore allows).