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

🔎 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

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

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

const wrapCallback = <K extends keyof SomeCallbacks>(
  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