RelTraitSet#plus uses FlatLists and ImmutableList. They have different hash algorithms, and they are all different classes from RelTraitSet. The RelTraitSet equality requires the other object must be RelTraitSet too, and the HashMap#get(key) uses key.equals() to check equality, instead of the other way. In case we pass RelTraitSet as the search key to cache, but the cached entry has key with FlatLists or ImmutableList, we may fail to find the cached RelTraitSet. Due to this, using == to check traitSet equality may fail, even they have same traits.