permit with a class which is not a subtype is allowed

forax at forax at
Tue Sep 3 18:42:04 UTC 2019

----- Mail original -----
> De: "Brian Goetz" <brian.goetz at>
> À: "Remi Forax" <forax at>
> Cc: "Maurizio Cimadamore" <maurizio.cimadamore at>, "amber-spec-experts" <amber-spec-experts at>
> 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.


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