throw new UnsupportedOperationException("checkUserLoginable is not supported.");
    }

    /**
     * Checks the credential using the provided credential checker.
     * This method is not supported in the Fess implementation.
     *
     * @param checker the credential checker to use
     * @throws UnsupportedOperationException always thrown as this method is not supported
     */
    @Override

                }),
        (accum, t) -> {
          /*
           * We assign these to variables before calling checkNotNull to work around a bug in our
           * nullness checker.
           */
          K key = keyFunction.apply(t);
          V newValue = valueFunction.apply(t);
          accum.put(
              checkNotNull(key, "Null key for input %s", t),

      }
    }

    /*
     * TODO(cpovirk): `return (V) removedValue` once our checker permits that.
     *
     * (We promoted a class of warnings into errors because sometimes they indicate real problems.
     * But now we need to "undo" some instance of spurious errors, as discussed in
     * https://github.com/jspecify/checker-framework/issues/8.)
     */
    return removedValue == null ? null : (V) removedValue;
  }

                }),
        (accum, t) -> {
          /*
           * We assign these to variables before calling checkNotNull to work around a bug in our
           * nullness checker.
           */
          K key = keyFunction.apply(t);
          V newValue = valueFunction.apply(t);
          accum.put(
              checkNotNull(key, "Null key for input %s", t),

  @CanIgnoreReturnValue
  public final <A extends Appendable> A appendTo(A appendable, @Nullable Object[] parts)
      throws IOException {
    @SuppressWarnings("nullness") // TODO: b/316358623 - Remove suppression after fixing checker
    List<?> partsList = Arrays.<@Nullable Object>asList(parts);
    return appendTo(appendable, partsList);
  }

  /** Appends to {@code appendable} the string representation of each of the remaining arguments. */

  @CanIgnoreReturnValue
  public final <A extends Appendable> A appendTo(A appendable, @Nullable Object[] parts)
      throws IOException {
    @SuppressWarnings("nullness") // TODO: b/316358623 - Remove suppression after fixing checker
    List<?> partsList = Arrays.<@Nullable Object>asList(parts);
    return appendTo(appendable, partsList);
  }

  /** Appends to {@code appendable} the string representation of each of the remaining arguments. */

      checkElementsNotNull(elements, n);
      ensureRoomFor(n);
      /*
       * The following call is not statically checked, since arraycopy accepts plain Object for its
       * parameters. If it were statically checked, the checker would still be OK with it, since
       * we're copying into a `contents` array whose type allows it to contain nulls. Still, it's

      } catch (Exception | Error unsafeFailure) { // sneaky checked exception
        thrownUnsafeFailure = unsafeFailure;
        // Catch absolutely everything and fall through to AtomicReferenceFieldUpdaterAtomicHelper.
        try {
          helper = new AtomicReferenceFieldUpdaterAtomicHelper();
        } catch (Exception // sneaky checked exception
            | Error atomicReferenceFieldUpdaterFailure) {

      } catch (Exception | Error unsafeFailure) { // sneaky checked exception
        thrownUnsafeFailure = unsafeFailure;
        // Catch absolutely everything and fall through to AtomicReferenceFieldUpdaterAtomicHelper.
        try {
          helper = new AtomicReferenceFieldUpdaterAtomicHelper();
        } catch (Exception // sneaky checked exception
            | Error atomicReferenceFieldUpdaterFailure) {

    this.inputFuture = checkNotNull(inputFuture);
    this.function = checkNotNull(function);
  }

  @Override
  @SuppressWarnings({
    "CatchingUnchecked", // sneaky checked exception
    "nullness", // TODO(b/147136275): Remove once our checker understands & and |.
  })
  public final void run() {
    @RetainedLocalRef ListenableFuture<? extends I> localInputFuture = inputFuture;
    @RetainedLocalRef F localFunction = function;