Unsafe Map overload spoils direct property access on parenthesized assignment expression

还没有人认领这个 Issue。

### 🔎 Search Terms

map overload any unknown unsafe weakmap set binary assignment lazy init

### 🕗 Version & Regression Information

- This is the behavior in every version I tried

### ⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.0-dev.20250326#code/DYUwLgBAxghlAWIBcECyMAOAeAzmATgJYB2A5gDQQD2ARgFYhRgB8EAPhAK7EAmIAZiRA8A3ACgx-bk0JViEMCDwBGABQBrEAE8UeImQCUEAN5iI0OXgj4lnYJAC8EVbAQgIAfg9PiIAO5omKoGBgB0pOAa2gbi5gD0cebmAHoeZtbgnPjyNjh2YOIAvhJSxDJyCkpgAExROhB6JKRGpuauiJ7eEL4B6BjBsRbEVrn5EE7tIOGRmlox6QlJEKnpNmBZObb2RWJAA

### 💻 Code

```ts
let cache: Map | undefined;

function test1(key: string) {
const result = (cache ??= new Map()).get(key);
// ^? const result: any
return result;
}

function test2(key: string) {
cache ??= new Map();
const result = cache.get(key);
// ^? const result: object | undefined
return result;
}
```

### 🙁 Actual behavior

First `result` gets spoiled as `any`

### 🙂 Expected behavior

I'd expect both `result`s to be `object | undefined`

### Additional information about the issue

This bug reports an issue related to `Map` but, tbf, it's not unique to `Map` and a more general solution would have to be found to properly fix this. The problem is how `checkBinaryLikeExpressionWorker` create a union of left and right types here - without taking into consideration that the assignment target might already overlap with the right type. That union type gets reduced with `UnionReduction.Subtype` and `.get` gets read from it - instead of the assigned to reference on the left side.

使用链接的 TypeScript Playground 重现 test1 和 test2 之间的差异，然后检查 checkBinaryLikeExpressionWorker；报告指出它负责形成赋值操作数的联合类型。跟踪该联合类型在解析 .get 之前是如何被缩减的。完成的标准是，带括号的赋值访问能够像单独的赋值形式一样推断出 object | undefined，并为报告中的示例添加回归覆盖。

由索引模型根据 Issue 内容生成。