False positive in SwitchDefault in Java 21 CaseLabelTree

[oxkitsune]

There's a false positive in the SwitchDefault checker when using Java 21's pattern matching in switch cases.

Simple example

sealed interface Foo {
  final class Bar implements Foo {}
}

class Test {
  void f(Foo i) {
    switch (i) {
      case Foo.Bar bar -> {}
    }
  }
}

This results in:

warning: [SwitchDefault] The default case of a switch should appear at the end of the last statement group
            case Foo.Bar bar -> {}
            ^
    (see https://errorprone.info/bugpattern/SwitchDefault)
  Did you mean to remove this line?

This happens because of the way default cases are detected in the SwitchDefault checker at Line 47. CaseTree#getExpression() will return null for the default case, but also for the new CaseLabelTree cases that are used for the pattern matching.

In JDK21 there's CaseTree#getLabels() which will return a list containing a DefaultCaseLabelTree, which could be used to differentiate between these cases.
I haven't been able to find a reliable method to differentiate between these two without relying on JDK21 features. Perhaps we could do some naive string checks? E.g.

Optional<? extends CaseTree> maybeDefault =
    tree.getCases().stream()
        .filter(c -> state.getSourceForNode(c).contains("default"))
        .findAny();

This example will need to handle the different syntax for CaseKind.RULE and CaseKind.STATEMENT ofcourse.

[rickie]

@cushon Do you have an opinion / suggestion on how to best solve this one?

We also have problems with this one internally.

[cushon]

I'm working on a fix for this, thanks for the report.

The tentative plan is to use reflection to call CaseTree#getLabels(), we're already using reflection to access other new APIs related to the changes to switch.