On Tue, 30 Jun 2026 19:21:35 GMT, Jan Lahoda <[email protected]> wrote:
>> Consider source files like this:
>>
>> $ cat /tmp/ValueClass.java
>> value class ValueClass {}
>> $ cat /tmp/TestValue.java
>> public class TestValue {
>> ValueClass c;
>> }
>>
>>
>> When compiled like this:
>>
>> $ javac --enable-preview --release 28 -Xlint:preview /tmp/ValueClass.java
>> /tmp/ValueClass.java:1: warning: [preview] value classes are a preview
>> feature and may be removed in a future release.
>> value class ValueClass {}
>> ^
>> 1 warning
>> $ javac --enable-preview --release 28 -Xlint:preview -classpath /tmp
>> /tmp/TestValue.java
>> warning: [preview] class file for /tmp/ValueClass.class uses preview
>> features of Java SE 28.
>> 1 warning
>>
>>
>> Note that while the `TestValue.class` is marked as preview, there's no
>> warning on the use of `ValueClass`. This is not aligned with the JEP 12
>> wording:
>>
>>> When compiling with preview features enabled, any source code reference to
>>> (i) a class or interface declared using a preview language feature, ...,
>>> causes a preview warning.
>>
>> Currently javac marks the classfile preview using an explicit test in the
>> `ClassWriter`, but that does not seem quite right. This PR proposes to mark
>> value classes as `Preview.declaredUsingPreviewFeature == true`, which should
>> automatically produce a warning, and mark the appropriate classfile as
>> preview.
>>
>> For the example above:
>>
>> $ javac --enable-preview --release 28 -Xlint:preview -classpath /tmp
>> /tmp/TestValue.java
>> warning: [preview] class file for /tmp/ValueClass.class uses preview
>> features of Java SE 28.
>> /tmp/TestValue.java:2: warning: [preview] class ValueClass is declared using
>> a preview feature, which may be removed in a future release.
>> ValueClass c;
>> ^
>> 2 warnings
>>
>>
>> Note this will also report the warning for `Integer`, when preview is
>> enabled:
>>
>> $ cat /tmp/Test.java
>> public class Test {
>> Integer i;
>> }
>> $ javac /tmp/Test.java
>> $ javac --enable-preview --release 28 -Xlint:preview /tmp/Test.java
>> warning: [preview] class file for /@/java.base/java/lang/Integer.sig uses
>> preview features of Java SE 28.
>> /tmp/Test.java:2: warning: [preview] class Integer is declared using a
>> preview feature, which may be removed in a future release.
>> Integer i;
>> ^
>> 2 warnings
>>
>>
>> ---------
>> - [x] I confirm that I make this contribution in accordance with the
>> [OpenJDK Interim AI Policy](https://openjdk.org/legal/ai).
>
> Jan Lahoda has updated the pull request incrementally with one additional
> commit since the last revision:
>
> Adjusting tests to the recent change.
@lahodaj
Your change (at version 988c4b51478ccce1db7b585a100b339b4864e483) is now ready
to be sponsored by a Committer.
-------------
PR Comment: https://git.openjdk.org/valhalla/pull/2601#issuecomment-4857381802
