type DiscriminateMessageType<
  A,
  T extends BookingMessageNotification['bookingMessageType']
> = A extends BookingMessageNotification ?
  T extends A['bookingMessageType'] ? A : never
: never;

  asMsg<T extends BookingMessageNotification['bookingMessageType']>(type: T): DiscriminateMessageType<BookingMessageNotification, T> {
    return this._entry as any;
  }