Cached adapters do not allow adaptees to be garbage collected

The caching functionality of the adaptation framework is a 1:1 port of the old traits.protocols code, and is provided mostly for backward compatibility. The problem with it is that the cache will not be cleared unless you take care of cleaning the ‘adaptee’ trait when your adapters are deleted.

If you cache the adaptation from adaptee to an adapter, the adapter is keeping a reference to the adaptee, and so the cache cannot be freed once the adaptee and the adapters disappear. WeakRefs to the adapters do not work, as one of the use cases is keeping adapter state across adaptation (so the adapter should stay around as long as the original adaptee is).