TypeScript can't infer type of default parameters about typescript HOT 2 OPEN

This is not specific to generator functions. It's related to inferring type parameters from unannotated parameters with inferred types from initializers (TS playground):

declare function fnDerive<A extends unknown[]>(fn: (...args: A) => void): A;

const result = fnDerive(function (a = 0, b = "") {});
//    ^? const result: [a?: unknown, b?: unknown]

declare function fnDerive2<A, B>(fn: (a: A, b: B) => void): [A, B];

const result2 = fnDerive2(function (a = 0, b = "") {});
//    ^? const result2: [unknown, unknown]

It's also just related to how there is a contextual signature available here. This touched on what I improved here: #56506 . This PR didn't break it anyhow when it comes to the case presented here but it also didn't improve it.

So the question here is also - how both situations should be differentiated? There are situations in which the contextual parameter type should be preferred. Perhaps it could be done based on checkMode & CheckMode.Inferential, or maybe based on the uninstantiated contextual signature when the parameter's type is a type variable.

I'd love to experiment with this but I have a long list of things to experiment with so no promises 😅

from typescript.

This is not specific to generator functions.

Updated title and search terms to reflect this~

from typescript.