documentation request: standard sample for Collection#toArray implementation

I'm a bit puzzled on the way to implement Collection#toAraray().

  protected static class Flat1List<T>
      extends AbstractFlatList<T>
      implements ComparableList<T> {
    private final T t0;

    public @PolyNull Object[] toArray(Flat1List<@PolyNull T> this) {
      Object[] res = new Object[1];
      res[0] = t0;
      return res;
    }

results in

FlatLists.java:398: error: [assignment.type.incompatible] incompatible types
in assignment.
      res[0] = t0;
               ^
  found   : T extends @Initialized @Nullable Object
  required: @Initialized @NonNull Object

    public @PolyNull Object[] toArray(Flat1List<@PolyNull T> this) {
      return new Object[]{t0};
    }

results in

FlatLists.java:397: error: [return.type.incompatible] incompatible types in r
eturn.
      return new Object[]{t0};
             ^
  type of expression: @Initialized @Nullable Object @Initialized @NonNull []
  method return type: @Initialized @PolyNull Object @Initialized @NonNull []

It looks like the workaround is to use, however, it is awkward. Am I doing something wrong?

    public @PolyNull Object[] toArray(Flat1List<@PolyNull T> this) {
      return new Object[]{castNonNullWithoutAssert(t0)};
    }

Start by locating Flat1List and its toArray implementation in FlatLists.java, then reproduce the two Checker Framework diagnostics shown in the issue. Review the Collection#toArray contract and the existing castNonNullWithoutAssert workaround. Done looks like documenting a standard sample implementation and clarifying whether that workaround is required.