RFR: 8361570: Incorrect 'sealed is not allowed here' compile-time error [v2]

[Michael Bien]

On Tue, 8 Jul 2025 09:28:58 GMT, Jan Lahoda <jlahoda at openjdk.org> wrote:

>> Consider code like this:
>> 
>> $ cat /tmp/T.java 
>> import java.lang.ref.*;
>> public class T {
>>     public static void main(String[] args) {
>>         new WeakReference<>(null) {};
>>     }
>> }
>> 
>> 
>> Compiling this with JDK 25/26 leads to:
>> 
>> $ ./jdk-25/bin/javac /tmp/T.java
>> /tmp/T.java:4: error: modifier sealed not allowed here
>>         new WeakReference<>(null) {};
>>                                   ^
>> 1 error
>> 
>> 
>> Which does not make much sense.
>> 
>> The reason for this is as follows:
>> - the type parameter for `WeakReference` is marked with `@jdk.internal.RequiresIdentity`, and the `WeakReference`'s constructor has a parameter whose type is this type parameter.
>> - as a consequence, this parameter has internally in javac flag `REQUIRES_IDENTITY`. Note this flag has currently the same `long` value as `SEALED`, as the value is reused to mean different things for different Symbol kinds.
>> - when creating the anonymous class, javac creates a constructor, copying the `REQUIRES_IDENTITY` together with the constructor's parameter
>> - then javac goes on and checks whether the flags on the parameter are correct. And it sees the value for `SEALED` is set, and reports the error
>> 
>> Ultimately, I don't think we can reuse the value of `SEALED` to mean different things (and the same for all other similar cases). This PR assigns a different value for `SEALED`, and tries to add a test that strives to hopefully prevent similar cases in the future by saying that no `Flags` in `ExtendedStandardFlags` can be reused.
>
> Jan Lahoda has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fixing tests.

kudos to @lahodaj and colleagues who are working on the fixes - you guys are super fast. This PR appeared ~10h after the Apache NetBeans smoke test found it, the CCE issue (JDK-8361445) before that is also already fixed.

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PR Comment: https://git.openjdk.org/jdk/pull/26181#issuecomment-3049157819