'in' does not remove undefined from indexed value (with noUncheckedIndexedAccess)

Nessuno ha ancora preso questa issue.

### 🔎 Search Terms

in operator, noUncheckedIndexedAccess

### 🕗 Version & Regression Information

Tested 5.8.2 and 5.9.0-dev.20250310

### ⏯ Playground Link

https://www.typescriptlang.org/play/?noUncheckedIndexedAccess=true&ts=5.8.2#code/MYewdgzgLgBAhgJwQLhmArgWwEYFMEDaAujALwwECMANDAEy0DMRA3AFCiSwCWAJgB6oMOfGXrtuAMxgAKPvxjcw8JAEoYAbzYwdMTtBiC0WPAjGJC81tt36QAG1wA6eyADmM-qvYBfNkA

### 💻 Code

With an array:
```ts
const arr: number[] = [1, 2, 3];
const idx: number = 2;
if (idx in arr) {
const x: number = arr[idx];
console.log(x);
}
```

With a map/object:
```ts
const map: Record = { a: 1 };
const key: string = "a";
if (key in map) {
const x: number = map[key];
console.log(x);
}
```

### 🙁 Actual behavior

```
Type 'number | undefined' is not assignable to type 'number'.
Type 'undefined' is not assignable to type 'number'.
```

### 🙂 Expected behavior

Expected 'in' operator to guarantee that the result of indexing is not `undefined` in this branch.

### Additional information about the issue

_No response_

Start with the linked TypeScript Playground reproduction using noUncheckedIndexedAccess and both the array and Record examples. Trace how the type checker narrows indexed access inside the `in` branches. Done means both examples allow assigning the indexed values to number without weakening the option’s existing behavior.

Scritto dal modello di indicizzazione a partire dal testo della issue.