Runtime errors when implementing non-sealed java interface

[PhoenixmitX]

Compiler version

• 3.7.1
• 3.nightly

Minimized code

#!/usr/bin/env scala
//> using scala "3.7.1"
// //> using scala 3.nightly also tried with nightly
//> using repository "https://repo.papermc.io/repository/maven-public/"
//> using dep "com.velocitypowered:velocity-api:3.4.0-SNAPSHOT"

import com.velocitypowered.api.command.RawCommand
import com.velocitypowered.api.command.RawCommand.Invocation

class CrashCommand() extends RawCommand:
	override def execute(invocation: Invocation): Unit =
		()

object Main:
	def main(args: Array[String]): Unit =
		val command = CrashCommand()

Not tried, but i think the java library can be minimized to:

public non-sealed interface RawCommand extends InvocableCommand {}
public sealed interface InvocableCommand permits RawCommand {}

Output

When RUNNING this code the jvm throws an error

Downloading 2 dependencies
Compiling project (Scala 3.7.1, JVM (21))
Compiled project (Scala 3.7.1, JVM (21))
Exception in thread "main" java.lang.IncompatibleClassChangeError: class Crash$_$CrashCommand cannot implement sealed interface com.velocitypowered.api.command.InvocableCommand
        at java.base/java.lang.ClassLoader.defineClass1(Native Method)
        at java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:1027)
        at java.base/java.security.SecureClassLoader.defineClass(SecureClassLoader.java:150)
        at java.base/jdk.internal.loader.BuiltinClassLoader.defineClass(BuiltinClassLoader.java:862)
        at java.base/jdk.internal.loader.BuiltinClassLoader.findClassOnClassPathOrNull(BuiltinClassLoader.java:760)
        at java.base/jdk.internal.loader.BuiltinClassLoader.loadClassOrNull(BuiltinClassLoader.java:681)
        at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:639)
        at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:188)
        at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:526)
        at Crash$_$Main$.main(Crash.sc:16)
        at Crash_sc$.main(Crash.sc:35)
        at Crash_sc.main(Crash.sc)

Expectation

no error is thrown

[som-snytt]

It works with Scala 2.

The minimization is subtle. It requires a default method in the sealed interface.

I minimized from
https://github.com/PaperMC/Velocity/tree/dev/3.0.0/api/src/main/java/com/velocitypowered/api/command
which leverages several features (inner interfaces, generics, additional parents and permits).

public sealed interface Seal permits NonSeal {
    default int g() {
        return 42;
    }
}

and

public non-sealed interface NonSeal extends Seal {
}

then

class C() extends NonSeal

@main def test = C()

and

$ javac -d /tmp *.java && scala-cli run --server=false -S 3.7.2-RC1 -cp /tmp -d /tmp test.scala

noting

public class C implements Seal,NonSeal

Confirming that Scala 2 omits the Seal parent.

[som-snytt]

Maybe because

[[syntax trees at end of MegaPhase{elimErasedValueType, pureStats, vcElideAllocations, etaReduce, arrayApply, elimPolyFunction, tailrec, completeJavaEnums, mixin, lazyVals, memoize, nonLocalReturns, capturedVars}]]
package <empty> {
  @SourceFile("test.scala") class C() extends Object, NonSeal {
    super()
    def g(): Int = super[Seal].g()
  }
}

[som-snytt]

BTW the workaround is -Xmixin-force-forwarders:false.