[jdk17] RFR: 8268871: Adjust javac to updated exhaustiveness specification [v2]

[Vicente Romero]

On Thu, 24 Jun 2021 10:57:55 GMT, Jan Lahoda <jlahoda at openjdk.org> wrote:

>> The current updates spec for pattern matching in switch:
>> http://cr.openjdk.java.net/~gbierman/jep406/jep406-20210608/specs/patterns-switch-jls.html
>> 
>> tweaks the definition of exhaustive switches so that it include handling special types, like intersection types, sealed subclasses of sealed types, like:
>> 
>> 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";
>>         });
>>   }
>> }
>> 
>> 
>> etc. This patch attempts to tweak javac to work according to this specification. It builds covered types based on what types are covered, i.e. if all subtypes of a sealed type are covered, the sealed type itself is covered, and this is done transitively, so that in the example above, `D` and `E` will mark `C` as covered, and then `B` and `C` will mark `A` as covered, and as the type if `a` is `A`, it means the switch is exhaustive.
>
> Jan Lahoda has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Improving tests, and performance of exhaustive switch detection.

lgtm

-------------

Marked as reviewed by vromero (Reviewer).

PR: https://git.openjdk.java.net/jdk17/pull/78