Unbound type parameter leak when calling union of methods in some circumstances

[jcalz]

🔎 Search Terms

generic type parameter, union of functions, leak unbound escaped rogue fugitive

🕗 Version & Regression Information

• This is the behavior in every version I tried (back to 4.2 anyway), and I reviewed the FAQ for entries about leaky generic

⏯ Playground Link

Furnished

💻 Code

interface Err<T> {
    f<U>(a: (err: T) => U): Err<U>;
}
interface Ok<T> {
    f(a: (err: T) => unknown): Err<T>;
}
declare const e: Err<0> | Err<1> | Ok<2>;
const e2 = e.f(e => e);
//    ^? const e2: Err<U> | Err<2> | Err<0 | 1 | 2>
//      what is U? --> ^

🙁 Actual behavior

The type of e2 is a union that includes Err<U>, where U is an unbound type parameter.

🙂 Expected behavior

I expect type parameters to stay in their scopes. As for what Err<U> should actually be... I think, uh, maybe it should be another copy of Err<0 | 1 | 2> or just omitted from the union?

Additional information about the issue

This is a minimal example derived from a Stack Overflow question.

Not sure if #43961 and #55467 are related, or if those are different manifestations of type parameter escape.