`Extract` not narrowing type union in mapped type passed as generic

🔎 Search Terms

Extract, type union

🕗 Version & Regression Information

• This changed between versions 5.33 and 5.4

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.4.2#code/C4TwDgpgBAysBOBLAdgcwNIRAZwDwBUA+KAXigFEAPBAQwGNhcBrLAewDMoBvAWACgogqAG0AClBRQWIDlHxQa2OWIC6UCNQjIAJkuwIUqKAH4oogFxRkEAG4R4KyzWQh+AXwA0UfUjSF+-HSsyPpQAEYANqysYKRQBB6EABQAXpZwvhhYeEQAlKTEvAJQQSHAUOyWPoZxKe4BfCjA9uz00Ira3PyCAI4A7hBVBmj1fPwREOU06cNZOLgd-nxAA

💻 Code

type StringKeys<T> = Extract<keyof {
    [P in keyof T as T[P] extends string ? P: never]: any
}, string>

const bloop = <T,>(z: StringKeys<T>) => {
  const f: string = z
}

interface asd {
  qwe: string
}

let a: StringKeys<asd>

🙁 Actual behavior

z is not assignable to string in the bloop function

🙂 Expected behavior

z should be assignable to string, because we extract string-extending keys

Additional information about the issue

This was working just fine on 5.3.3, and continues to work when the generic is concretized (hence the let at the bottom of the example)