----- Mail original ----- > De: "Brian Goetz" <brian.go...@oracle.com> > À: "Remi Forax" <fo...@univ-mlv.fr> > Cc: "Maurizio Cimadamore" <maurizio.cimadam...@oracle.com>, > "amber-spec-experts" <amber-spec-experts@openjdk.java.net> > Envoyé: Mardi 3 Septembre 2019 18:13:58 > Objet: Re: permit with a class which is not a subtype is allowed
>> And you have the second effect which is, if the permit clause raise a warning >> when the class doesn't exist, it logically also means that the case clause >> of a >> switch on type also raises a warning when the class doesn't exist. >> > > This is true, but remember such things can happen in different files. > The real question is whether the naming of a type in a permits clause is > a _use_ of that type. (Clearly, naming a type in a case clause is a > use.) So while it would be impossible for a switch to proceed without > being able to find all the types it names, the same is not necessarily > true for the declaration. Until Java 9, every mention of a name not at a declaration site was a use, it was relaxed in Java 9 in module-info in case of a cross module reference because you may not want to compile several modules at the same time (and as far as i know Maven still is not able to do that). We have decided that a closed hierarchy should refer to types inside the same module, so it should be a use. For the switch on type, the current equivalent of a switch on type is a visitor, so people will expect that they will be able to retrofit a visitor to a switch on type. In a visitor, the expression of each case is isolated in its own method so if the type taken as parameter is not present, it's not a problem, the method can not be called (technically it even better because it can be called with null and it will still work). So at runtime, i think its reasonable to not raise an IncompatibleClassChange if one of the type of the case doesn't exist. So we are exactly in a similar case as with the permit clause, it should not be a problem at runtime but you want the compiler to consider them as use at compile time. Rémi
