This seems reasonable to me. So, spec consequences:
- sealed, non-sealed illegal on enums
- enums can implement sealed types
- said permission to extend pushes down to constants, including the anonymous
classes of nontrivial constants
> On Oct 11, 2019, at 6:59 AM, Maurizio Cimadamore
> <maurizio.cimadam...@oracle.com> wrote:
>
> I think an enum declaration is 'morally final' in the sense that, while it
> can't really be marked with ACC_FINAL (because there might be constants which
> extend from it), the user cannot subclass the enum. Everything weird you can
> do with an enum, remains _inside_ the enum declaration bubble, which I think
> makes mixing enums and sealed interface pretty safe. It is also lucky that we
> can't say 'final enum' - meaning that I would also extend it to the other
> keywords - that is, you can't put sealed, non-sealed on an enum.
>
> Regarding the 'anonymous enum constant' issue you raise how is that different
> from:
>
> sealed interface Y permits Bar, Baz {}
>
> class Bar implements Y {}
>
> ... new Bar() {}
>
> In this case, I don't think you break exhaustiveness in the same way you do
> if you allow anonymous implementations of Y.
>
> Clients will be assuming that Y is either a Bar or a Baz, and the fact that
> some of the Bars are anonymous instance is immaterial to this.
>
> Unless I misunderstood what you were trying to say. If not, I think my
> reasoning here would be to:
>
> 1) allow enums to implement sealed interfaces
> 1b) do not allow sealed, non-sealed modifiers on an enum (e.g. do the same as
> with final)
> 2) allow anonymous enum constants inside the enums in (1) - as they can't
> break exhaustiveness for clients
>
> Maurizio
>
>
> On 11/10/2019 04:02, Tagir Valeev wrote:
>> Hello!
>>
>> Sorry if this was already discussed, but what about enums extending
>> sealed interfaces? E.g.:
>>
>> sealed interface X permits Foo {}
>> enum Foo implements X { // can we do this?
>> A {}, // and what about this? Here we have an additional subclass at
>> runtime. Or we should explicitly declare "non-sealed enum Foo" to
>> allow this?
>> B,
>> C
>> }
>>
>> With best regards,
>> Tagir Valeev.
>>
>> On Thu, Oct 10, 2019 at 3:46 PM Maurizio Cimadamore
>> <maurizio.cimadam...@oracle.com> wrote:
>>>
>>> On 10/10/2019 01:50, Brian Goetz wrote:
>>>> Right. We already restrict anon and lambda instances of the sealed
>>>> type. Not only can't we stably write down their types in the PS
>>>> attribute, but even if we could, it's so easy to accidentally lose
>>>> exhaustiveness.
>>> This is a very good point; if I have type T = A | B | C, but then I have
>>> 'anonymous' Ts flying around, all switches assuming A|B|C are no longer
>>> exhaustive.
>>>
>>> Maurizio
>>>
