Typesafe omit typescript (TS 3.1.6)

Omit.ts

// type-safe omit
export function omit<T extends Object, K extends string | number | symbol>(obj: T, k: K): Omit<T, K> {
  const a = { ...(obj as Object) } as T;
  if (k in a) {
  	delete (a as any)[k];
  }
  return a as Omit<T, K>;
}

type A = {
	foo: string,
	bar: number,
};

type B = {
	foo: string,
	beep: string
};

type C = {
	bar: number,
	boop: string,
};

type D = A | C;

// correctly works
function shouldWork(a: A): B {
	const b: { foo: string } = omit(a, 'bar');
	return {
		...b,
		beep: a.bar.toString(),
	};
}

// correctly fails
function shouldFail1(a: A): B {
	const b: { foo: string, bar: number } = omit(a, 'bar');
	return {
		...b,
		beep: a.bar.toString(),
	};
}

// correctly fails
function shouldFail2(a: A): B {
	return {
		...a,
		beep: a.bar.toString(),
	};
}

// doesn't work but should :(
function shouldWork2(d: D): B | undefined {
	const b: { foo: string } | { boop: string } = omit(a, 'bar');
	return 'foo' in b
		? {
			...b,
			beep: a.bar,
		}
		: undefined;
}