Description
(discovered in https://bugzilla.mozilla.org/show_bug.cgi?id=1934205 )
In the proposal, ForInOfStatement definition is modified and the using handling is added into ForDeclaration with [Using] parameter, where the parameter is set only after a negative lookahead for for ofusing of:
https://arai-a.github.io/ecma262-compare/?pr=3000&id=sec-for-in-and-for-of-statements&secAll=true
ForInOfStatement[Yield, Await, Return] :
for ( [lookahead ≠ let [] LeftHandSideExpression[?Yield, ?Await] in Expression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return]
for ( var ForBinding[?Yield, ?Await, +Pattern] in Expression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return]
for ( ForDeclaration[?Yield, ?Await, ~Using] in Expression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return]
for ( [lookahead ∉ { let, async of }] LeftHandSideExpression[?Yield, ?Await] of AssignmentExpression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return]
for ( var ForBinding[?Yield, ?Await, +Pattern] of AssignmentExpression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return]
for ( [lookahead ≠ using of] ForDeclaration[?Yield, ?Await, +Using] of AssignmentExpression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return]
[+Await] for await ( [lookahead ≠ let] LeftHandSideExpression[?Yield, ?Await] of AssignmentExpression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return]
[+Await] for await ( var ForBinding[?Yield, ?Await, +Pattern] of AssignmentExpression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return]
[+Await] for await ( [lookahead ≠ using of] ForDeclaration[?Yield, ?Await, +Using] of AssignmentExpression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return]
ForDeclaration[Yield, Await, Using] :
LetOrConst ForBinding[?Yield, ?Await, +Pattern]
[+Using] using [no LineTerminator here] ForBinding[?Yield, ?Await, ~Pattern]
[+Using, +Await] await [no LineTerminator here] using [no LineTerminator here] ForBinding[?Yield, +Await, ~Pattern]
So, for (using of cannot become a prefix of the for-in/for-of with using declaration, but it can be a prefix of for-of with the using being an identifier in LeftHandSideExpression.
On the other hand, the proposal doesn't touch the ForStatement definition, and the using handling is added into LexicalDeclaration unconditionally.
https://arai-a.github.io/ecma262-compare/?pr=3000&id=sec-for-statement&secAll=true
ForStatement[Yield, Await, Return] :
...
for ( LexicalDeclaration[~In, ?Yield, ?Await] Expression[+In, ?Yield, ?Await]opt ; Expression[+In, ?Yield, ?Await]opt ) Statement[?Yield, ?Await, ?Return]
LexicalDeclaration[In, Yield, Await] :
LetOrConst BindingList[?In, ?Yield, ?Await, +Pattern] ;
UsingDeclaration[?In, ?Yield, ?Await]
[+Await] AwaitUsingDeclaration[?In, ?Yield]
UsingDeclaration[In, Yield, Await] :
using [no LineTerminator here] BindingList[?In, ?Yield, ?Await, ~Pattern] ;
AwaitUsingDeclaration[In, Yield] :
CoverAwaitExpressionAndAwaitUsingDeclarationHead[?Yield] [no LineTerminator here] BindingList[?In, ?Yield, +Await, ~Pattern] ;
This means ForStatement matches for (using of = obj;;);.
Is this expected?
TypeScript doesn't recognize the syntax, and
Babel, esbuild, Chromium, and SpiderMonkey throw syntax error.
Apparently the negative lookahead is applied also to ForStatement in all of them.
If for (using of = obj;;); should throw SyntaxError, the ForStatement syntax should be modified to have a negative lookahead, with propagating the existence of using to LexicalDeclaration, or maybe replacing that part with something similar to ForDeclaration.
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