Crash in CFG construction for case null in switch expression

[msridhar]

See uber/NullAway#831. Here is the code snippet:

static <T extends Metric<?>> GroupByQuery<T> toGroupByQueryWithExtractor(
      GroupBy groupBy, GroupByResultExtractor<T> extractor) {
    return switch (groupBy) {
      case OWNER -> new GroupByOwnerQuery<>(extractor);
      case CHANNEL -> new GroupByChannelQuery<>(extractor);
      case TOPIC -> new GroupByTopicQuery<>(extractor);
      case TEAM -> new GroupByTeamQuery<>(extractor);
      case null -> throw new IllegalArgumentException("GroupBy parameter is required");
    };
  }

The top of the crash stack is:

  Caused by: java.lang.ClassCastException: class com.sun.tools.javac.tree.JCTree$JCLiteral cannot be cast to class com.sun.source.tree.IdentifierTree (com.sun.tools.javac.tree.JCTree$JCLiteral and com.sun.source.tree.IdentifierTree are in module jdk.compiler of loader 'app')
        at org.checkerframework.nullaway.dataflow.cfg.builder.CFGTranslationPhaseOne$SwitchBuilder.casesAreExhaustive(CFGTranslationPhaseOne.java:2565)
        at org.checkerframework.nullaway.dataflow.cfg.builder.CFGTranslationPhaseOne$SwitchBuilder.build(CFGTranslationPhaseOne.java:2344)
        at org.checkerframework.nullaway.dataflow.cfg.builder.CFGTranslationPhaseOne.visitSwitchExpression17(CFGTranslationPhaseOne.java:585)
        at org.checkerframework.nullaway.dataflow.cfg.builder.CFGTranslationPhaseOne.scan(CFGTranslationPhaseOne.java:548)
        at org.checkerframework.nullaway.dataflow.cfg.builder.CFGTranslationPhaseOne.visitReturn(CFGTranslationPhaseOne.java:3389)

Based on the JCLiteral type I'm guessing the issue is with case null. I'll work on putting together a test case, just wanted to report in case the root cause is obvious.

[smillst]

(Note using null in a case statement is a preview feature of Java 20, so I just used Java 21 to reproduce.)
Here's a testcase:

import org.checkerframework.checker.nullness.qual.Nullable;

public class Issue6173 {

  static Object toGroupByQueryWithExtractor2(
      @Nullable GroupBy groupBy) {
    return switch (groupBy) {
      case OWNER -> new Object();
      case CHANNEL -> new Object();
      case TOPIC -> new Object();
      case TEAM -> new Object();
      case null -> throw new IllegalArgumentException("GroupBy parameter is required");
    };
  }

  public enum GroupBy {
    OWNER,
    CHANNEL,
    TOPIC,
    TEAM;
  }
}

[smillst]

We also need to fix the Nullness Checker so that is allows this case, which I'm working on.