A review of JEP 401 showed that javac's prototype implementation
differs from the draft JEP in a few ways.

These are captured in the bugs:

JDK-8267918 - Javac tolerates synchronizing on a primitive reference type
JDK-8267910 - Javac fails to implicitly type abstract classes as implementing IdentityObject
JDK-8267907 - Withdraw support for the experimental option -XDallowValueMemberCycles
JDK-8267843 - Duplicate error when primitive class instance is passed around before being fully initialized
JDK-8267841 - A primitive class that implements Cloneable is tolerated by javac
JDK-8267835 - Javac tolerates vacuous chaining to super constructor from primitive class constructor
JDK-8267965 - Javac disallows calls to super.{hashCode(), equals(), toString()} on a primitive instance receiver