Possible refinement for sealed classes

[Guy Steele]

> On Apr 1, 2021, at 9:23 AM, Brian Goetz <brian.goetz at oracle.com> wrote:
> 
> ...
> Without trying to paint the bikeshed, this is a pretty simple extension to sealing:
> 
>     sealed interface WithCheese __refines Sandwich { }
> 
> This says that WithCheese is sealed to either classes that extend Sandwich, or other interfaces that __refines Sandwich.  As a bonus, default methods in WithCheese can refer to methods in Sandwich, since it must always be the case that `this instanceof Sandwich`.  
> 
> Essentially, I want to put the `permits` list on Sandwich, and have `WithCheese` and friends delegate their permits to `Sandwich`, rather than having each interface enumerate their subtypes.

Maybe “__refines” should be spelled “extend”?

If an interface J extends a class C, it means that it is a compile-time error if class D implements interface J but class D is not a subclass of C.

Such a constraint might be useful even if C is not sealed.

—Guy

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