abstract class Value {
  readonly valueType: number;
  abstract value(): any;
}

class NullValue extends Value {
  valueType = 0;

  value(): any {
    return null;
  }
}

class NumberValue extends Value {
  valueType = 1;

  constructor(readonly num: number) { super(); }

  value(): any {
    return this.num;
  }
}

function process(node: Value) {
  if (node instanceof NullValue) {
    console.log('Null!')
  } else if (node instanceof NumberValue) {
    // Produces: TS2339: Property 'num' does not exist on type 'never'.
    console.log('NumberValue', node.num);
  }
}