Description of the false positive
"Call to equals() comparing incomparable types AbstractSet and ImmutableSet."

This actually works just fine, because AbstractSet and ImmutableSet only use the Set<> interface's semantics, and do not rely on what implementation of Set they are.

URL to the alert on the project page on LGTM.com

https://lgtm.com/projects/g/google/error-prone/snapshot/30c381a82e01a40b3f7f255ed05d0857bcf0d35e/files/core/src/main/java/com/google/errorprone/bugpatterns/UnnecessaryDefaultInEnumSwitch.java?sort=name&dir=ASC&mode=heatmap#L96