TypeScript can't infer type of default parameters

🔎 Search Terms

infer default parameters
function wrapper
function factory

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ
  • I tried down to version 4.0.3 (since in the earlier version Generator type is not generic)

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/CYUwxgNghgTiAEAzArgOzAFwJYHtVNQBEQYsA3EAHgEF4QAPDEVYAZ3jQGtUcB3VANoBdADTwASgD4AFACh4CggC540gHQbYAc1YrqASngBeSfADizElAw4YlLj35ipI2fpUO++APQAqX-AAtjhgnCDA8HAYyDD4vt4A3LKyiEQk5CDSKOjYePC+0lDG8AAMYgBGxQDkVYYA3vLwYHisOBAgahA4WtINivBQrvDe3vAAegD8jQrl8iPjUwoAvvpJK0lAA

💻 Code

declare function fnDerive<A extends unknown[], R>(
    fn: (...args: A) => Generator<unknown, R>,
): unknown /** mocked return */;

fnDerive(function *(a = 0, b = '') {
  console.log({
    a,
 // ^?
    b
 // ^?
  });
});

🙁 Actual behavior

The type of a and b is unknown.

🙂 Expected behavior

The type of a and b should be number and string

Additional information about the issue

I was working with a library gensync when I hit this issue, and I extracted the minimum reproduction out of it.

Note that if I don't use default parameters, typescript can infer the a and b types correctly:

declare function fnDerive<A extends unknown[], R>(
    fn: (...args: A) => Generator<unknown, R>,
): unknown /** mocked return */;

fnDerive(function *(a?: number | undefined, b?: string | undefined) {
  console.log({
    a,
 // ^?
    b
 // ^?
  });

  a ??= 0;
  b ??= '';
  console.log({
    a,
 // ^?
    b
 // ^?
  });
});

Commencez par le TypeScript Playground lié et la reproduction minimale de fnDerive, puis suivez la manière dont les paramètres par défaut sont gérés lors de l’inférence de fonctions génériques. C’est terminé lorsque l’exemple infère a comme number et b comme string, tandis que l’exemple équivalent avec un paramètre explicitement optionnel continue de fonctionner correctement.