Unsafe Map overload spoils direct property access on parenthesized assignment expression

Nessuno ha ancora preso questa 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.

Riproduci la differenza tra test1 e test2 usando il TypeScript Playground collegato, quindi esamina checkBinaryLikeExpressionWorker, che il report identifica come responsabile della formazione dell’unione degli operandi di assegnazione. Traccia come viene ridotta tale unione prima della risoluzione di .get. Il lavoro è completato quando l’accesso all’assegnazione tra parentesi inferisce object | undefined come la forma di assegnazione separata, con una copertura di regressione per l’esempio riportato.

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