A contextually-typed signature coming from correlated union can't be assigned back to source of its contextual type

まだ誰も着手していません。

### 🔎 Search Terms

correlated union signature union map mapped type index

### 🕗 Version & Regression Information

- This is the behavior in every version I tried

### ⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.0-dev.20250508#code/C4TwDgpgBAIhBmBLAdo4iD2yDOUC8UA3gFBRRYBiGGAXFANoCGATgOZ3ICuAtgEYTMAugG5S5ZACEWdJmzrZgzFKxHEAvqOKhIUAMoZuEAMKMANqd6MAxgGtcBEmXoBpKCig2IIDPFgIUaJg4gnQAFAB0kSys2HRwSKjoWNguggCU+AB8UABuGIgAJqIaxMRWycBQAO7MjGAm5pa2+FAAPK4QAB7AEMgFuJ7evvqGDRbWdpmhYtgYnMxWEHQjxmbjttgANGLA0RDAywarjRNbYoN0ztsZeNmOUOU4lVZrTTYts-OL9IOqO3vAH5eQQtCJRNjYG53MRkRC+UIAWUYwAAFuFan0DKEMtkAAzhACsGXuZDIzH282QD1eEzB4WikNEpKgajEJRKxHgnGQViSVJqdQAguZQi8ThtDqMaRs0pLjus7EQxI8FNVamBIAUWoQ1FBGLgVmM3tgmVB4BhmFBQqZ9h4vG4qWKFZClcyBfVpTZRZ6tmq6prNnaQHqBl4fHojkbTmlTayyRTmPz1ZrisQgA

### 💻 Code

```ts
type Definitions = {
onFoo: [arg: number];
onBar: [arg: string];
};

type SomeCallbacks = {
[K in keyof Definitions]: (...args: Definitions[K]) => void;
};

const wrapCallback = (
source: SomeCallbacks,
target: SomeCallbacks,
key: K,
) => {
const callback = source[key];

target[key] = (...args) => {
if (Math.random() > 0.5) {
return callback(...args);
}
};
};

function wrapAll(callbacks: SomeCallbacks): SomeCallbacks {
const wrapped = {} as SomeCallbacks;
for (let key in callbacks) {
wrapCallback(callbacks, wrapped, key as keyof SomeCallbacks);
}
return wrapped;
}
```

### 🙁 Actual behavior

It fails to recognize that this function is assignable to `target[key]` despite the fact that `args` are contextually-typed by that `target[key]`

### 🙂 Expected behavior

I'd expect this to typecheck just fine

### Additional information about the issue

_No response_

まず、提供された correlated-union と mapped-type の例を使って、リンク先の TypeScript Playground でエラーを再現します。コンテキストから型付けされた callback の型チェック経路と、target[key] への代入を追跡します。完了条件は、報告されている型の関係を弱めることなく、例が期待どおりに型チェックを通ることです。

索引モデルが issue の本文から書いたものです。