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

Commencez par reproduire l’erreur dans le TypeScript Playground lié en utilisant l’exemple fourni d’union corrélée et de type mappé. Suivez le chemin de vérification des types pour le callback typé contextuellement et son affectation à target[key] ; le travail est terminé lorsque l’exemple est vérifié comme prévu, sans affaiblir les relations de types signalées.