Sealed local interfaces

[Dan Smith]

> On Aug 17, 2020, at 8:07 AM, Remi Forax <forax at univ-mlv.fr> wrote:
> 
>  static void foo() {
>    sealed interface I {}
> 
>    record Foo implements I {}
>  }
> 
> but this code does not compile because Foo is a local class and a local class can not implement a sealed interface.

This scenario is not directly covered by the specs, because local interfaces are a feature of the Records JEP, while 'sealed' is a feature of the Sealed Classes JEP. (If we'd noticed the interaction, we could have specified Sealed Classes in terms of the "Local Static Interfaces and Enum Classes" changes. But if those changes are finalized in 16, the point will be moot.)

However, we *did* make a deliberate choice that a local *class* cannot be 'sealed' at all, and that restriction should naturally apply to local interfaces, too:

"It is a compile-time error if a local class declaration contains any of the access modifiers public, protected, or private (6.6), or any of the modifiers static (8.1.1), sealed or non-sealed (8.1.1.2)."

I kind of think this is overly-restrictive, but it was a safe way to sidestep all the questions that arise due to classes being "unavailable" from a certain scope. I think it's worth thinking more carefully about this in the next iteration.