`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.

Beginne mit der reproduzierten Klasse im Playground-Link und untersuche checkGrammarPrivateIdentifierExpression in src/compiler/checker.ts rund um die isForInStatement-Bedingung. Bestätige die derzeit für for (const a in #b) erzeugte Diagnose und vergleiche sie mit dem erwarteten Fehler für private Bezeichner. Erledigt ist die Aufgabe, wenn die ungültige Form mit der erwarteten Diagnose abgelehnt wird und das Verhalten durch die Regressionstests des Compilers abgedeckt ist.