Type narrowing not working correctly with Uppercase<string> & Lowercase<string>

还没有人认领这个 Issue。

### 🔎 Search Terms

"narrowing lowercase" "narrowing uppercase"

### 🕗 Version & Regression Information

- This is the behavior in every version I tried, and I reviewed the FAQ for entries about Common Bugs and Type System Behavior

### ⏯ Playground Link

https://www.typescriptlang.org/play/?#code/DYUwLgBAxg9grgOzAJwJ4GEYBMQC4IBEAQgKIEQA+hAMgKrlUEBy1BA3AFAewIDOkAB2QgAZgEsAHhAC8hAEYgCAOjAxaAgSGToAhrxAAKAJQQ9EdZuRQ9IADz9kYhAHMAfJw5iREA0NGSZaVliMkoqP3EpIOC6BnDhSMDglgITAG8OCCzoeCQ0TBwZCAjJTgB6MqyAPQ4KrIAVVE0IAHILLWt9exQnNxaIMV4IBBhIPV4xZwQdOVAIVXmmkFaQhhp6SkIUlqVayohG5paHXv7B4dHTXgmpmbmFsCWV0jWCWM3mVh2OAF8gA

### 💻 Code

```ts
let countryCode: "BE" | "LU" | "NL";

const prefix = "be".toUpperCase() as Uppercase;

if (prefix === "BE" || prefix === "LU" || prefix === "NL") {
countryCode = prefix;
// ^
// Type 'Uppercase' is not assignable to type '"BE" | "LU" | "NL"'.
// Type 'string' is not assignable to type '"BE" | "LU" | "NL"'.
}
```

### 🙁 Actual behavior

TypeScript cannot narrow down an `Uppercase` or `Lowercase` type after explicitly comparing against specific values.

### 🙂 Expected behavior

TypeScript should accept the code above as `"BE" | "LU" | "NL"` is strictly a sub-type of `Uppercase`.

### Additional information about the issue

_No response_

首先复现链接的 TypeScript Playground 示例，并比较字面量比较之后 Uppercase 和 Lowercase 的行为。跟踪导致赋值错误的类型缩窄路径。当列出的 "BE" | "LU" | "NL" 检查将值缩窄到足以使赋值成功的程度，并同时考虑等效的小写行为时，即视为完成。

由索引模型根据 Issue 内容生成。