Why is this switch over sealed types not exhaustive?

[Brian Goetz]

This looks like a bug.  D and E together cover C (since C is sealed *and 
abstract*), and B and C cover A (same reason.)  If either A or C were 
not abstract, you'd need a case for those.

On 6/15/2021 12:31 PM, Cay Horstmann wrote:
> Hi, I was surprised that the following switch expression "does not 
> cover all possible input values", according to JDK build 17-ea+26-2439.
>
> sealed abstract class A permits B, C {}
> final class B extends A {}
> sealed abstract class C extends A permits D, E {}
> final class D extends C {}
> final class E extends C {}
>
> public class Test {
>    public static void main(String[] args) {
>       A a = new D();
>       System.out.println(
>          switch (a) {
>             case B x -> "B";
>             case D x -> "D";
>             case E x -> "E";
>          });
>    }
> }
>
> However, the more combersome
>
>          switch (a) {
>             case B x -> "B";
>             case C c -> switch (c) {
>                case D x -> "D";
>                case E x -> "E";
>             };
>          }
>
> is deemed exhaustive. Is that a bug or does it work as designed?
>
> Thanks,
>
> Cay
>