Iterator dynamic class incorrectly determined

[daewok]

It seems that when determining the dynamic class of an iterator a nonsensical result is sometimes returned. This can be reproduced by loading cl-containers and evaluating:

(make-iterator '(1 2 3) :circular t :transform #'sqrt)

which returns an iterator of class CIRCULAR-ITERATOR-MIXIN-AND-TRANSFORMING-ITERATOR-MIXIN-AND-LIST-ITERATOR

And then evaluating

(make-iterator '(1 2 3) :circular t)

which also returns an iterator of class CIRCULAR-ITERATOR-MIXIN-AND-TRANSFORMING-ITERATOR-MIXIN-AND-LIST-ITERATOR. Additionally, any attempt to use this iterator immediately errors because the transform function is nil.

This seems to be a result of the success criteria for find-existing-subclass being too weak. It simply finds any class that subclasses all supers, even if that class was defined to include other mixins that aren't in the list of supers.

I actually discovered this while working with the dynamic-classes package, but it's much easier to demonstrate incorrect behavior using cl-container. I'm not sure how to fix this, but if I manage to come up with a way I'll submit a pull request.