if (lowerBounds.length > 0) {
      @SuppressWarnings("unchecked") // T's lower bound is <? extends T>
      TypeToken<? extends T> bound = (TypeToken<? extends T>) of(lowerBounds[0]);
      // Java supports only one lowerbound anyway.
      return bound.getSubtype(subclass);
    }
    throw new IllegalArgumentException(subclass + " isn't a subclass of " + this);
  }