this-shenanigans.ts

interface Base {
    // @ts-ignore
    other: number extends this['val'] ? number : string;
}

interface Other<T = undefined> {
    val?: T extends undefined ? number : T;
}

interface Test<T = undefined> extends Base, Other<T> {}

const t = 0 as never as Test<string>;
t.other;
// ^? t.other is `string`

const tt = 0 as never as Test
tt.other;
// ^? tt.other is `number`