There's an interesting interaction between IdentityObject and abstract superclasses of inline classes that might be worth leaning into.
--- The "status quo" (inasmuch as one exists): An inline class can extend a class if it, and all of its supers, 1) are abstract, 2) declare no instance fields, and 3) have "empty" <init> methods. These properties represent a new kind of abstract class—call it a "light" abstract class. Changing a "light" abstract class to be "heavy" is a binary incompatible change. Separately, we have the IdentityObject interface, which is implicitly attached to all non-inline concrete classes. An abstract class might also be able to implement IdentityObject explicitly, and doing so would also disqualify it from being an inline class super. A struggle in this story is getting programmers to care about whether their classes are "heavy" or "light", since even though it's an important property, it's easy to overlook (there's no syntax for it, and in many cases, there are no immediate effects). --- Alternative story: An inline class must not extend IdentityObject through any of its superclasses. (That's it.) A non-inline class implicitly implements IdentityObject if it 1) is concrete, 2) declares an instance field, or 3) has a non-empty <init> method. Abstract classes can also explicitly implement IdentityObject. Changing a class so that it implements IdentityObject is a binary incompatible change. Now we have a highly-visible concept (IdentityObject) that programmers should generally be aware of anyway, and they should readily understand a difference between abstract classes that implement IdentityObject and those that don't. --- I think I like the alternative story. It feels simpler. One reason to avoid it is if we think there's potentially value in a "light" abstract class concept that is independent of IdentityObject. For example, maybe some other feature could build on the idea of a superclass that requires no initialization, without tying that to the topic of object identity. I'm having trouble envisioning a use case, though. Another reason to avoid it is if we want IdentityObject to be limited to concrete classes—no explicit implementing it allowed. If the alternative story is the one we want, it implies that the "empty <init>" JVM feature should be part of Inline Classes, not a separate thing we deliver earlier—because it's directly tied to IdentityObject.
