example.ts

type Base = {
    x: number,
};

type Derived = Base & {
    y: number,
};

let to = <T>(x: T): T => x;

let f = (): Base => {
    return { x: 5 } as Derived; // No error.
};

let g = (): Base => {
    return <Derived>{ x: 5 }; // No error.
};

let h = (): Base => {
    return to<Derived>({ x: 5 }); // Propery 'y' is missing!
};