@SuppressWarnings("nullness") // TODO: b/423853632 - Remove after checker is fixed.
    @Override
    public @Nullable V computeIfPresent(
        K key, BiFunction<? super K, ? super @NonNull V, ? extends @Nullable V> remappingFunction) {
      synchronized (mutex) {
        return delegate().computeIfPresent(key, remappingFunction);
      }
    }

    @Override
    public @Nullable V compute(
        K key,

    assertThrows(
        UnsupportedOperationException.class, () -> unmod.computeIfAbsent(3, k -> k + "three"));
    assertThrows(UnsupportedOperationException.class, () -> unmod.computeIfPresent(4, (k, v) -> v));
    assertThrows(UnsupportedOperationException.class, () -> unmod.compute(4, (k, v) -> v));
    assertThrows(UnsupportedOperationException.class, () -> unmod.merge(4, "four", (k, v) -> v));