8366968: Exhaustive switch expression rejected by for not covering all possible values

[lahodaj]

Consider case like (from the bug):

class Demo {

    sealed interface Base permits Special, Value {}

    non-sealed interface Value extends Base {}

    sealed interface Special extends Base permits SpecialValue {}

    non-sealed interface SpecialValue extends Value, Special {}

    static int demo(final Base base) {
        return switch (base) {
            case final Value value -> 0;
            // Uncommenting the following line will make javac accept this program
            //case final Special value -> throw new AssertionError();
        };

    }

}

This fails to compile:

/tmp/Demo.java:12: error: the switch expression does not cover all possible input values
        return switch (base) {
               ^
1 error

Note there is no instance of Special that would not be an instance of Value as well. I.e. covering Value will catch all input.

Also, note that if case Value is replaced with case SpecialValue, javac also compile the code:

            case final Value value -> 0;
=>
            case final SpecialValue value -> 0;

$ ~/tools/jdk/jdk-25/bin/javac /tmp/Demo.java
$

Which shows the problem: replacing a type with a super type should not cause the switch to stop to be exhaustive (i.e. the switch is exhaustive for SpecialValue, but replacing it with its super type Value, the switch is no longer exhaustive for javac).

javac contains a piece of code that searches through subtypes to handle diamond class hierarchies like the one above. But, when it searches for subtypes, it does a search through permitted subtypes of the type. And since Value is not sealed, this search will not find SpecialValue, and javac won't see the switch to be exhaustive.

The proposal herein is to broaden the search, and consider all transitive permitted subtypes of the selector type, and filter subtypes of the current type from this set (if the current type is abstract, if it is not abstract, we can't do the subtype search at all). This should, I think, include all relevant subtypes.

---

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• JDK-8366968: Exhaustive switch expression rejected by for not covering all possible values (Bug - P4)

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