Iterator<? extends T> iterator) {
    if (iterator instanceof PeekingImpl) {
      // Safe to cast <? extends T> to <T> because PeekingImpl only uses T
      // covariantly (and cannot be subclassed to add non-covariant uses).
      @SuppressWarnings("unchecked")
      PeekingImpl<T> peeking = (PeekingImpl<T>) iterator;
      return peeking;
    }

*   `tf.function`/AutoGraph:
    *   AutoGraph is now available in `ReplicaContext.merge_call`,
        `Strategy.extended.update` and `Strategy.extended.update_non_slot`.
    *   Experimental support for shape invariants has been enabled in
        `tf.function`. See the API docs for
        `tf.autograph.experimental.set_loop_options` for additional info.
    *   AutoGraph error messages now exclude frames corresponding to APIs