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

まだ誰も着手していません。

### 🔎 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)

--noUncheckedIndexedAccess を使用した、リンク先の TypeScript Playground の再現コードから始め、issue にある 2 つのインデックスアクセス式を比較してください。issue ではリポジトリのファイルもテストも指定されていません。ユニオンに対するインデックスアクセスがどのように解決されるかを追跡し、その後、2 つ目の式に undefined が含まれることを示すカバレッジを追加して、関連するコンパイラテストを実行してください。

索引モデルが issue の本文から書いたものです。