JavaParsers doesn't support JEP 445: Unnamed Classes and Instance Main Methods

[xuwei-k]

Reproduction steps

• Scala 2.13.12
• JDK 21
• https://openjdk.org/jeps/445
• xuwei-k/Scala-JEP-445@c96fe40
• https://github.com/xuwei-k/Scala-JEP-445/actions/runs/6268842307/job/17024415920

A.java

void main() {
  System.out.println("hello");
}

B.scala

class B

build.sbt

javacOptions ++= Seq(
  "-Xlint:preview",
  "--enable-preview",
  "--release",
  scala.util.Properties.javaSpecVersion
)

scalaVersion := "2.13.12"

project/build.properties

sbt.version=1.9.6

Problem

sbt -J--enable-preview compile

https://github.com/scala/scala/blob/e67d287447c09720468f8bebcb0302bd92d75f43/src/compiler/scala/tools/nsc/javac/JavaParsers.scala#L1074

[error] /home/runner/work/Scala-JEP-445/Scala-JEP-445/A.java:1:6: illegal start of type declaration
[error] void main() {
[error]      ^
[error] one error found
[error] (Compile / compileIncremental) Compilation failed

[xuwei-k]

https://openjdk.org/jeps/512 finalize in JDK 25

[som-snytt]

It kind of makes you not want to support Java.

[lrytz]

Support for simpler main methods seems to just work with Scala bytecode

$ cat T.scala
class T {
  def main(): Unit = println("hi")
}

$ sc T.scala
> scala compile --server=false -S 2.13 -d . -release 8 T.scala

$ java -version
openjdk version "25-ea" 2025-09-16

$ sr T
> scala run -S 2.13 -cp . -M T
hi

[lrytz]

Took a quick look. Spec diff (via https://docs.oracle.com/javase/specs/).

Quirks

---

// T.java
class K
void main() { }

This defines classes T and T$K. Commenting the main method turns the class K top-level. So the nesting in final class T { ... } has to happen at the end of parsing the compilation unit.

---

Compact compilation units have an implicit import module java.base which brings more symbols in scope than the import java.lang.* in ordinary compilation units.

import module x is not supported in Scala 2.13 nor Scala 3 (scala/scala-dev#529).

---

With the version I tested (25-ea+22-2667), a compact compilation unit cannot referenced in joint compilation (as expected by the JEP). But the resulting Filename.class can be referenced in separate compilation.

---

Perhaps the best way to support this JEP is to set unit.body = EmptyTree when encountering a sparse compilation unit. What ever is in there cannot be referenced anyway.