Guava's "immutable collections" are very popular https://github.com/google/guava/wiki/ImmutableCollectionsExplained and it's not a good idea to fight their advertised notion of "immutable".
No generic container class can promise s'marks-style immutability (until valhalla perhaps?) so it's not that useful a concept in this domain. We like to think of Optional as an immutable value based class but you can't stop anyone who really wants to mutate the contained element from creating an Optional<AtomicReference<Object>> We could do a better job of clarifying consequences of element mutation. E.g. do we ever say that hash based collections/maps are broken if elements are ever mutated in such a way that their hash code changes while they are in the collection/map?
