Switch coverage with multiple branches

[Dan Smith]

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.