Dependent typescript solution

script2.ts

type FooOrBar = "foo" | "bar";

type MaybeFoo<T extends FooOrBar> = T extends "foo" ? {y: number} : {}


type Box<T extends FooOrBar> =
  T extends "foo" ? { x: "foo"} & MaybeFoo<T> :
  T extends "bar" ? { x: "bar"} & MaybeFoo<T> :
  never;

function foo(f: Box<FooOrBar>): number {
  if (f.x === "foo") {
    return f.y
  } else {
    return 4
  }
}