Parameters in functions with type parameters are not contextually-typed

[Andarist]

🔎 Search Terms

type parameters generic conextual parameter types context sensitive

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.0-dev.20250601#code/KYDwDg9gTgLgBAE2AYwDYEMrDsiA7AZ3gDM8AuOAHgBUAaOAQSgHMC5QZg8E2mp0AnpXR4BAPnoAlYDDEAKALAAoOHALBUxCnWWqARhAQCKcmAAsAlgW30AdPcysKTVgEo4AXjFwA4l2D8MNDColIy9HjAAG4BYsruXnBy9raO1owsBAne0jDKyqCQsDj4RHAQANaecKRyAN5wALbABATozMAUAETEEBBdcAC+9MQArnjIMBb4AFRy5HB4o416Ae51uiWEEKjAtqgQzKaWBLbNre3AEa7KgzdKBeDQ8LiE8HgQ1bUN520d3b1+kMRuNJtM8DNKAA5eQLKHrTavAg7PYHI7mKxnFp-K6Le53ZRAA

💻 Code

export declare const fn: <T, Args extends Array<any>, Ret>(
  self: T,
  body: (this: T, ...args: Args) => Generator<any, Ret, never>
) => (...args: Args) => Ret

export const ok = fn({ message: "foo" }, function*(n: number) {
  console.log(this.message, n) // ok
})

export const no = fn({ message: "foo" }, function*<N>(n: N) {
  console.log(this.message, n) // error
})

🙁 Actual behavior

The second case errors

🙂 Expected behavior

Both of those should work fine

Additional information about the issue

Given TS is able (at times) to infer new generic signatures based on generic function arguments (notice the return type of the failing case: const no: <N>(n: N) => void), it makes sense to assign contextual parameter types.