throw new UnsupportedOperationException();
    }

    @Override
    @CheckForNull
    /*
     * Our checker arguably should produce a nullness error here until we see @NonNull in JDK APIs.
     * But it doesn't, which may be a sign that we still permit parameter contravariance in some
     * cases?
     */
    public V computeIfPresent(

          forwardCmp = (Comparator) Ordering.natural();
        }
        result = comparator = reverse(forwardCmp);
      }
      return result;
    }

    // If we inline this, we get a javac error.
    private static <T extends @Nullable Object> Ordering<T> reverse(Comparator<T> forward) {
      return Ordering.from(forward).reverse();
    }

    @Override
    @ParametricNullness