Unions seem to sidestep `--noUncheckedIndexedAccess` and lose the possibility of undefined

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### 🔎 Search Terms

noUncheckedIndexedAccess, union, undefined

### 🕗 Version & Regression Information

- This is the behavior in every version I tried, and I reviewed the FAQ for entries about `--noUncheckedIndexedAccess`

### ⏯ Playground Link

[Playground link](https://www.typescriptlang.org/play/?noUncheckedIndexedAccess=true#code/MYewdgzgLgBGCuBbCAuGBvGBtA1m6ATgJZgDmAumgogEYCmBMAvjALwwCyAhlABYB0BLmAAmIRAAoAlDAA8MAAz8ArDAD8GGFzQBGZjDSYaaAEzMA3AChQkWITabtMAES86AG3chnzS5YD0-jAA7kR8MAC0EWAgAKpgwG7AOHQiAJKidAAeqQCCwMB0EBAw4Nbg0DC8iIgOEtx8gsJikjLySqoa1BD8XAYwhL0ygQNQxGQwAD5wSPSM0-CZAGYkqeW2Ibx97PU8AkKi4tJyiirqM8j9hFK9MCOEJKRTF3N+QA)

### 💻 Code

```ts
const nums: { [k: string]: number } = Math.random() < 0.5 ? { a: 1 } : { b: 2 };
const str = { a: "hello" }

// with --noUncheckedIndexedAccess on
const hmm = (Math.random() < 0.5 ? nums.a : str.a) // string | number | undefined
const wha = (Math.random() < 0.5 ? nums : str).a // string | number <-- 😕
```

### 🙁 Actual behavior

The type of `wha` is `string | number`, completely ignoring the possibility that it might be `undefined`, even though `--noUncheckedIndexedAccess` is enabled. Looks like unions of types with index signatures and known keys lose the `--noUncheckedIndexedAccess` behavior.

### 🙂 Expected behavior

`wha` should be of type `string | number | undefined`, just like `hmm`, since indexed access into a union should look like a union of indexed accesses.

### Additional information about the issue

This is related to #50474. It’s also related to #47531, which had specifically to do with `never[]` and so the focus was on avoiding `never[]` as opposed to investigating what happened to the indexed access. ( https://github.com/microsoft/TypeScript/issues/47531#issuecomment-1229522810 )

Ran into this when looking at [a Stack Overflow question](https://stackoverflow.com/q/79451704/2887218)

Inizia con la riproduzione collegata in TypeScript Playground usando --noUncheckedIndexedAccess e confronta le due espressioni di accesso indicizzato nell’issue. L’issue non indica alcun file o test del repository; ricostruisci come viene risolto l’accesso indicizzato per le unioni, quindi aggiungi una copertura che mostri che la seconda espressione include undefined ed esegui i test pertinenti del compilatore.

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