[sealed] Sealed local classes?

[Brian Goetz]

I agree its reasonable, but I don’t think the spec quite allows it yet, so we’ll put that on the list of things to be refined.

> On Oct 12, 2019, at 12:53 PM, Tagir Valeev <amaembo at gmail.com> wrote:
> 
> Yes, I just wanted to draw attention to this detail. The current spec
> draft doesn't cover this case explicitly. I agree: this solution is
> reasonable.
> 
> With best regards,
> Tagir Valeev.
> 
> On Fri, Oct 11, 2019 at 9:07 PM Brian Goetz <brian.goetz at oracle.com> wrote:
>> 
>> 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.cimadamore at 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.cimadamore at 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
>>>>> 
>>