`for (const x in #b)` is allowed

🔎 Search Terms

Private identifier, for...in

🕗 Version & Regression Information

• This changed between versions 4.5.5 and 4.6.4

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.5.0-dev.20240603#code/MYGwhgzhAECC0G8BQ1oGIBGAKAlIlq0AZgPYBO0WwJAdhAC7RjQCWN6Geyh0Avgf35A

💻 Code

class A {
  #b() {
    for (const a in #b) {
    }
  }
}

🙁 Actual behavior

No errors

🙂 Expected behavior

Error: Private identifiers are only allowed in class bodies and may only be used as part of a class member declaration, property access, or on the left-hand-side of an 'in' expression

Additional information about the issue

Noticed this while working on https://github.com/typescript-eslint/typescript-eslint/pull/9232. There's a very suspicious condition in code: https://github.com/microsoft/TypeScript/blob/5a4134470128a062a8297f404dfb3321f8f55798/src/compiler/checker.ts#L33699

    function checkGrammarPrivateIdentifierExpression(privId: PrivateIdentifier): boolean {
        if (!getContainingClass(privId)) {
            return grammarErrorOnNode(privId, Diagnostics.Private_identifiers_are_not_allowed_outside_class_bodies);
        }

        if (!isForInStatement(privId.parent)) {
            if (!isExpressionNode(privId)) {
                return grammarErrorOnNode(privId, Diagnostics.Private_identifiers_are_only_allowed_in_class_bodies_and_may_only_be_used_as_part_of_a_class_member_declaration_property_access_or_on_the_left_hand_side_of_an_in_expression);
            }

            const isInOperation = isBinaryExpression(privId.parent) && privId.parent.operatorToken.kind === SyntaxKind.InKeyword;
            if (!getSymbolForPrivateIdentifierExpression(privId) && !isInOperation) {
                return grammarErrorOnNode(privId, Diagnostics.Cannot_find_name_0, idText(privId));
            }
        }

        return false;
    }

I don't know what isForInStatement is supposed to do there. I was unable to find the commit that introduced this—git blame took me to #44648 but that didn't have this condition.

Commencez par la classe reproduite dans le lien Playground et examinez checkGrammarPrivateIdentifierExpression dans src/compiler/checker.ts autour de la condition isForInStatement. Confirmez le diagnostic actuellement produit pour for (const a in #b) et comparez-le avec l’erreur attendue concernant l’identifiant privé. C’est terminé lorsque la forme invalide est rejetée avec le diagnostic attendu et que le comportement est couvert par les tests de régression du compilateur.