exclusive-or.ts

type Neverize<T, U> = { [P in Exclude<keyof T, keyof U>]?: never };
type ExclusiveOr<T, U> = (T | U) extends object
  ? (Neverize<T, U> & U) | (Neverize<U, T> & T)
  : T | U;

interface S {
  value: string;
  string: string;
}

interface N {
  value: number;
  number: string;
}

function what(x: ExclusiveOr<S, N>) {
  return x.value;
}

what({ value: 'asdf', string: 'asdf' });
what({ value: 5, string: 'asdf' });

what({ value: 5, number: 'asdf' });
what({ value: 'asdf', number: 'asdf' });

what({ value: 'asdf', string: 'asdf', number: 'asdf' });