Furthermore on interfaces, has there been discussion around finalization of default implementations?

Like non-overridable default implementations?

Yes, exactly. This’d give us what are essentially mix-ins and better observe the OCP. You can get there partially through class delegation but it feels awkward.

but then we get very close to an

abstract class

(minus internal state) and back at the multiple-inheritance problems

interfaces have already solved the problem of “which method gets called” with

super<T>

We have default methods, but we can’t specify whether those methods are virtual. There must be a limitation under the hood for this.