On Aug 22, 2023, at 12:29 PM, John Rose <[email protected]> wrote:
If abstract classes which are super classes of value classes can now have fields, does that encourage developers to adopt patterns which rely on pointer polymorphism and forfeit flattening? The concern here is less about can we do this but more about should we do it? I think we should consider this feature for a future release of Valhalla. There is nothing special about this feature that would make it defeat the VM in its usual job of optimizing whatever the user throws at it. As usual inlining will probably be sufficient, and if not there are other tricks we can play. I'll add that I think the moral hazard of writing polymorphic code is equally present in interfaces (default methods) and fieldless abstract classes (concrete methods that don't mention fields). I'm not seeing that dynamic significantly changing in the presence of fields. Does this hold together with our story on implicit constructors (generate both an implicit_creation attribute and a method in the class)? Probably, but that’s a question Dan can answer more surely than me. Yeah, the "I allow implicit instance creation" metadata is, at the class file level, orthogonal to constructors. The flag enables the VM to create all-zeros instances without running any code. The constructor bodies determine what happens when a constructor is explicitly invoked.
