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

[Aggelos Biboudis]

On Fri, 21 Mar 2025 14:11:18 GMT, Jan Lahoda <jlahoda at openjdk.org> wrote:

> 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 should. 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.

Seems good.

-------------

Marked as reviewed by abimpoudis (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/24155#pullrequestreview-2709587739