8348928: Check for case label validity are misbehaving when binding patterns with unnamed bindings are present

[lahodaj]

Consider code like:

public class Exp0 {
    public static void main(String... args) {
        Object o = "";
        record R(int i, String s) {}

        switch (o) {
            case Integer _, R(int i, String _) -> {}
            default -> {}
        }
    }
}

This compiles OK, but it shouldn't. The JLS says:

It is a compile-time error for a case label to have more than one case pattern and declare any pattern variables (other than those declared by a guard associated with the case label).

Some similar cases leads to a javac crash:

$ cat Exp2.java 
public class Exp2 {
    public static void main(String... args) {
        Object o = "";
        record R(int i, String s) {}

        switch (o) {
            case Integer _:
            case R(int i, String _):
                System.err.println(i);
                break;
            default: break;
        }
    }
}

$ jdk-24/bin/javac Exp2.java
An exception has occurred in the compiler (24-internal). Please file a bug against the Java compiler via the Java bug reporting page (https://bugreport.java.com) after checking the Bug Database (https://bugs.java.com) for duplicates. Include your program, the following diagnostic, and the parameters passed to the Java compiler in your report. Thank you.
java.lang.AssertionError
        at jdk.compiler/com.sun.tools.javac.util.Assert.error(Assert.java:155)
        at jdk.compiler/com.sun.tools.javac.util.Assert.check(Assert.java:46)
        at jdk.compiler/com.sun.tools.javac.jvm.Items$LocalItem.<init>(Items.java:392)
        at jdk.compiler/com.sun.tools.javac.jvm.Items.makeLocalItem(Items.java:133)
        at jdk.compiler/com.sun.tools.javac.jvm.Gen.visitIdent(Gen.java:2350)
        at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCIdent.accept(JCTree.java:2711)
...
printing javac parameters to: /tmp/javac.20250321_150757.args

The root cause here is a bug when determining whether or not a given case label contains bindings - the "has-bindings" flag gets cleared when _ is used. The "has-bindings" flags must remain set despite seeing unnamed binding variables.

---

Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Commit message must refer to an issue
• Change requires CSR request JDK-8348933 to be approved

Issues

• JDK-8348928: Check for case label validity are misbehaving when binding patterns with unnamed bindings are present (Bug - P3)
• JDK-8348933: Check for case label validity are misbehaving when binding patterns with unnamed bindings are present (CSR)

Reviewers

• Adam Sotona (@asotona - Reviewer)
• Aggelos Biboudis (@biboudis - Reviewer)

Reviewing

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[bridgekeeper]

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[mlbridge]

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• 00: Full (888e87b9)

[asotona]

Looks good to me.

[biboudis]

Seems good.