The `options` arg of `Temporal.ZonedDateTime.prototype.toLocaleString()` currently accepts "illegal" option `timeZone`

@RyanCavanaugh is already working on this.

Since Sep 19, 2026.

• #64346 by @copilot-swe-agent — open

### ⚙ Compilation target

ESNext

### ⚙ Library

ESNext

### Missing / Incorrect Definition

https://github.com/microsoft/TypeScript/blob/0b832e12235cc3db14a4be6501827c2f8a8a8569/tsc/internal/bundled/libs/lib.esnext.temporal.d.ts#L320

### Sample Code

TypeScript allows the following code, when it should not.

```TypeScript
Temporal.Now.zonedDateTimeISO("America/Chicago").toLocaleString(undefined, { timeZone: "Europe/Berlin" });
```

#### Result

| Engine | Error |
| -------------- | ------------------------------------------------------------------------------------------------------------------ |
| JavaScriptCore | `TypeError: ZonedDateTime.toLocaleString does not accept a timeZone option; the ZonedDateTime's time zone is used` |
| SpiderMonkey | `TypeError: can't set option timeZone when Temporal.ZonedDateTime.toLocaleString is used` |
| V8 | `TypeError: Invalid time zone specified: America/Chicago` |

### Documentation Link

According to [MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Temporal/ZonedDateTime/toLocaleString#options):

> The timeZone option must not be provided, as it is automatically set to be the date-time's [timeZoneId](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Temporal/ZonedDateTime/timeZoneId).

The type definitions should be changed to

```typescript
declare namespace Temporal {
// ...
interface ZonedDateTimeToLocaleStringOptions extends Omit {}
// ...
interface ZonedDateTime {
// ...
toLocaleString(locales?: Intl.LocalesArgument, options?: ZonedDateTimeToLocaleStringOptions): string;
}
}
```