Yes, in a garbage-collected language a ref cell is typically unhashable since the only identifying data about it is the pointer location, which could change bet

Yes, in a garbage-collected language a ref cell is typically unhashable since the only identifying data about it is the pointer location, which could change between calls to the hash function. However, you can still define equality pretty easily (just compare the pointer values). This is also my best example of something with an equivalence relation that is not comparable. I don't have a ready example of something that is comparable but not hashable, but it isn't uncommon in my experience to come across an opaque type whose author simply didn't expose enough information to hash it, while they did expose a compare function.