[sealed] Sealed local classes?

[Tagir Valeev]

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
> >>>
>