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
 // ^?
  });
});

Start with the linked TypeScript Playground and the minimal fnDerive reproduction, then trace how default parameters are handled during generic function inference. Done means the example infers a as number and b as string, while the equivalent explicitly optional-parameter example continues to behave correctly.