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).
-------------
Commit messages:
- 8387484: [lworld] javac should warn on use of value classes
Changes: https://git.openjdk.org/valhalla/pull/2601/files
Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=2601&range=00
Issue: https://bugs.openjdk.org/browse/JDK-8387484
Stats: 115 lines in 3 files changed: 87 ins; 9 del; 19 mod
Patch: https://git.openjdk.org/valhalla/pull/2601.diff
Fetch: git fetch https://git.openjdk.org/valhalla.git pull/2601/head:pull/2601
PR: https://git.openjdk.org/valhalla/pull/2601
