`using` transform throws when a for body binding shadows the for head binding

[JLHwung]

🔎 Search Terms

explicit resource management, using, await using, loop initializer, Symbol.dispose, Symbol.asyncDispose, iteration, shadows

🕗 Version & Regression Information

• This changed between versions ______ and _______
• This changed in commit or PR _______
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about https://github.com/microsoft/TypeScript/wiki/FAQ#common-bugs-that-arent-bugs
• I was unable to test this on prior versions because _______

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.0-dev.20250516#code/MYGwhgzhAEBiD29oG8C+BYAUAM3gJ2gAoBXCASwDsBzaXJebaAbQF0BKFLaaYeCiAC61E0ALzQKAUwDucRITYBuLKiA

💻 Code

class Foo {}
for (using foo of []) {
  const foo = new Foo();
}

🙁 Actual behavior

The transformed output throws "redeclaration of const foo "

🙂 Expected behavior

The transformed output should not throw.

Additional information about the issue

Babel is also affected by this issue: babel/babel#17318. We plan to wrap the for body within a new block when it contains shadowed binding.

[RyanCavanaugh]

This seems like a bizarre thing to write; we might just choose to error instead.

[JLHwung]

This example is also valid JS and TS so far has no complaints:

class Foo {}
for (const foo of []) {
  const foo = new Foo();
}

If TS decides to error instead, this will introduce inconsistency between const and using.

[RyanCavanaugh]

It seems like a likely code error to do that, though. You could have written

for ({} of []) {

instead

[JLHwung]

Yes, you are right. They are just valid pedantic examples. So I am calling for consistency. If TS thinks

for (using foo of []) {
  const foo = new Foo();
}

is bad and should throw an error (though currently the error is unintentional because of the transform result). Then the same error should be applied to

for (const foo of []) {
  const foo = new Foo();
}

as well.

The fact that the former is throwing while the latter is okay seems inconsistent.

[RyanCavanaugh]

@copilot: Issue a duplicate identifier error when this occurs in a target that requires using downleveling

[RyanCavanaugh]

@copilot forget that, let's do a proper fix. Do the same thing we would do in ES5 output where

for (let x of [1, 2, 3]) {
  const x = 4;
  let m = x;
}

becomes

for (var _i = 0, _a = [1, 2, 3]; _i < _a.length; _i++) {
    var x = _a[_i];
    var x_1 = 4;
    var m = x_1;
}

In other words, rename the inner shadowing variable.