`Extract` not narrowing type union in mapped type passed as generic about typescript HOT 4 OPEN

Conditional types are tricky as they often stay deferred in generic contexts. This version could somewhat easily work though (and it doesn't):

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

const bloop = <T,>(z: StringKeys<T>) => {
  const f: string = z; // errors but shouldnt
};

As a workaround, you can use this version:

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

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

from typescript.

I would guess it’s just a matter of the compiler knowing that T & string is always assignable to string regardless of what T is (by definition, as A & B is the set of values assignable to both)

from typescript.

That workaround works in my real codebase, so I'll cut over to that approach for now - is this a matter of the intersection (instead of Extract) causing non-deferred evaluation?

from typescript.

Bisects to #56742

from typescript.