----- Mail original ----- > De: "Brian Goetz" <brian.go...@oracle.com> > À: "Remi Forax" <fo...@univ-mlv.fr> > Cc: "amber-spec-experts" <amber-spec-experts@openjdk.java.net> > Envoyé: Jeudi 2 Avril 2020 23:31:23 > Objet: Re: Semantics of an empty PermittedSubtypes attribute for the VM
>>> At the language level, we won’t let you define a class with an empty set of >>> permitted types; you should define a final class instead. >>> >>> But, having the VM treat an empty PS attribute as if it were not there seems >>> wrong; I would think an empty PS attribute would be a class file error. >> For comparison, i believe the VM also allows attribute NestMembers or >> BootstrapMethods to be empty. >> > > True. But the semantics of these are not ambiguous, as they are with > PS. An empty "NestMembers" attribute, and no "NestMembers" attribute, > have the same effect -- no nest members. But an empty PermittedSubtypes > attribute means (in the absence of ACC_FINAL) anything goes. So we have > two things to be "consistent" with: > > - No PS means no restriction. And an empty PS should be like no PS, > right? So empty PS is "no restrictions." > - PS == list of permited subtypes, so empty PS is "no permitted subtypes." > > Given that these things are on a collision course, and there's no need > to have an empty PS, better to make an empty PS illegal. yes, make sense. Rémi
