8387484: [lworld] javac should warn on use of value classes by lahodaj · Pull Request #2601 · openjdk/valhalla

lahodaj · 2026-06-30T12:49:20Z

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

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Issue

JDK-8387484: [lworld] javac should warn on use of value classes (Bug - P4)

Reviewers

Maurizio Cimadamore (@mcimadamore - Committer) ⚠️ Review applies to d5313afa
Chen Liang (@liach - Committer) ⚠️ Review applies to d5313afa
Vicente Romero (@vicente-romero-oracle - Committer) ⚠️ Review applies to d5313afa

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bridgekeeper · 2026-06-30T12:50:59Z

👋 Welcome back jlahoda! A progress list of the required criteria for merging this PR into lworld will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

openjdk · 2026-06-30T12:51:45Z

@lahodaj This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8387484: [lworld] javac should warn on use of value classes

Reviewed-by: mcimadamore, liach, vromero

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 7 new commits pushed to the lworld branch:

71eb703: 8386160: [lworld] Post jdk-27+25 compiler/valhalla/inlinetypes/TestIntrinsics.java#id4 assertion
fa0f4a6: 8387464: [lworld] Increase memlimit and unproblemlist TestValueConstruction.java
2ae8e47: 8387454: [lworld] Update Strict fields JEP number
... and 4 more: https://git.openjdk.org/valhalla/compare/e07832f9d9b9c2cb8e4a446c4fdaccfa48da266f...lworld

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

As you do not have Committer status in this project an existing Committer must agree to sponsor your change. Possible candidates are the reviewers of this PR (@mcimadamore, @liach, @vicente-romero-oracle) but any other Committer may sponsor as well.

➡️ To flag this PR as ready for integration with the above commit message, type /integrate in a new comment. (Afterwards, your sponsor types /sponsor in a new comment to perform the integration).

mcimadamore

Nice improvement!

liach · 2026-06-30T12:54:56Z

     */
    public boolean declaredUsingPreviewFeature(Symbol sym) {
-        return false;
+        return sym.isValueClass();


Should this include records too given how its semantics are changed? Maybe in another RFE?

I am not completely certain, but I think this should not include records.

My understanding is that for records, only the implementation of the record's constructor changes, and that this is transparent(?) for the users of the record. I.e. the change not in the declaration/signatures, but in the implementation. If a class, e.g. PM, uses a preview feature inside an implementation only (i.e. not in a declaration), we don't taint another class, e.g. Use, that only uses PM.

For example:

$ cat PM.java public class PM { private boolean test(Object o) { return o instanceof byte b; } } $ cat Use.java public class Use { PM pm; } $ javac --enable-preview --source 28 -Xlint:preview PM.java Use.java PM.java:3: warning: [preview] primitive patterns are a preview feature and may be removed in a future release. return o instanceof byte b; ^ 1 warning

There's no warning for the use of PM in Use, and Use.class is not marked a preview classfile.

Even though PM uses a preview feature (primitive patterns), it only does so inside an implementation, and so the use of the preview feature is invisible to the use site, leading to no warning.

My understanding (possibly wrong) is that the change in record constructors is similar to the use of pattern matching in the implementation of PM.

mlbridge · 2026-06-30T13:00:54Z

Webrevs

01: Full - Incremental (988c4b51)
00: Full (d5313afa)

vicente-romero-oracle

lgtm

8387484: [lworld] javac should warn on use of value classes

d5313af

openjdk Bot added the rfr Pull request is ready for review label Jun 30, 2026

mcimadamore approved these changes Jun 30, 2026

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liach approved these changes Jun 30, 2026

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openjdk Bot added the ready Pull request is ready to be integrated label Jun 30, 2026

vicente-romero-oracle approved these changes Jun 30, 2026

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Adjusting tests to the recent change.

988c4b5

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8387484: [lworld] javac should warn on use of value classes#2601

8387484: [lworld] javac should warn on use of value classes#2601
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