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

还没有人认领这个 Issue。

### 🔎 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)

从使用 --noUncheckedIndexedAccess 的链接 TypeScript Playground 复现开始，并比较 issue 中的两个索引访问表达式。issue 没有指出任何仓库文件或测试；追踪联合类型的索引访问是如何解析的，然后添加覆盖，表明第二个表达式包含 undefined，并运行相关的编译器测试。

由索引模型根据 Issue 内容生成。