Switch coverage with multiple branches

[Remi Forax]

----- 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 ?

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