(<expression>,) is incorrectly treated as valid

[gibson042]

Environment: XS 10.5.0

Description
XS seems to interpret a comma-terminated expression inside expression parentheses as valid, which it should not per CoverParenthesizedExpressionAndArrowParameterList:

Supplemental Syntax

When processing an instance of the production
PrimaryExpression_{[Yield, Await]} : CoverParenthesizedExpressionAndArrowParameterList_{[?Yield, ?Await]}
the interpretation of CoverParenthesizedExpressionAndArrowParameterList is refined using the following grammar:

ParenthesizedExpression_{[Yield, Await]} : ( Expression_{[+In, ?Yield, ?Await]} )

Steps to Reproduce

// In value position
void (Date,);
// In a left-hand side
(Date,) = null;

Expected behavior
Each of the above statements should produce a SyntaxError.

Actual behavior
Both statements are accepted as equivalent to their valid counterparts that don't have the trailing comma.

[phoddie]

Hey @gibson042, thanks another detailed report. I'm able to duplicate your results. We'll take a look.

[phoddie]

(The two commits referenced above are mislabeled and do not apply to this report.)

[phoddie]

FYI – the issue is related to parsing arrow functions, strangely enough. The expression is (a, b, c, ) => {} (also with trailing comma inside parenthesis) is allowed because it is an arrow function. The parser needs a little more state to detect the error. This is expected to be resolved in XS 13.