[External] : Re: Switch coverage with multiple branches

[Dan Smith]

> On Jul 23, 2021, at 3:48 PM, Remi Forax <forax at univ-mlv.fr> wrote:
> 
> ----- Original Message -----
>> From: "daniel smith" <daniel.smith at oracle.com>
>> To: "amber-spec-experts" <amber-spec-experts at openjdk.java.net>
>> Cc: "Gavin Bierman" <gavin.bierman at oracle.com>
>> Sent: Samedi 24 Juillet 2021 00:28:08
>> Subject: Switch coverage with multiple branches
> 
>> An RFE for JEP 406 (or maybe bug fix? I haven't dug into what the spec says).
>> Can we make this compile?
>> 
>> public class SwitchCoverage {
>>   sealed interface A {}
>>   sealed interface B1 extends A {}
>>   sealed interface B2 extends A {}
>>   sealed interface C extends A {}
>>   final class D1 implements B1, C {}
>>   final class D2 implements B2, C {}
>> 
>>   void test(A arg) {
>>       int i = switch (arg) {
>>           case B1 b1 -> 1;
>>           case B2 b2 -> 2;
>>       };
>>   }
>> 
>> }
>> 
>> Output:
>> 
>> % -> `javahome 17`/bin/javac --enable-preview --release 17 SwitchCoverage.java
>> SwitchCoverage.java:10: error: the switch expression does not cover all possible
>> input values
>>       int i = switch (arg) {
>>               ^
>> Note: SwitchCoverage.java uses preview features of Java SE 17.
>> Note: Recompile with -Xlint:preview for details.
>> 1 error
>> 
>> The compiler wants to see a 'case C c', not realizing that the combination of B1
>> and B2 already covers C.
>> 
>> The use case might seem a impractically complex, but I think it's actually
>> pretty normal to want to define a universe of values (that's A), provide some
>> implementations (D1 and D2), and then categorize the implementations in
>> different dimensions (B1/B2 in one dimension, C in another). When I'm writing
>> my switch, I might only care about one of these dimensions.
> 
> Should it compile if C is declared as non-sealed ?

No. The analysis would see that

C = { D1, D2 }

and then note that

{ D1, D2 } is covered by { B1, B2 }