Could not recognize 'BoxedUnit' in Java as 'Unit'

[XYZboom]

Compiler version

3.6.3-RC2, 3.6.4-RC1-bin-20241231-1f0c576-NIGHTLY
(Tried in 2.13.15 too, but I don't know if it's necessary to report it to Scala 2 at the same time)

Minimized code

trait I0[T] {
  def func(): T
}
trait I1 extends I0[Unit] {
}
class A1 extends A0 {
  override def func(): Unit = ???
}

// Java File
// FILE: A0.java
import scala.runtime.BoxedUnit;

public class A0 implements I1 {
    @Override
    public BoxedUnit func() {
        return BoxedUnit.UNIT;
    }
}

Output

A0.scala:7:16
error overriding method func in class A0 of type (): scala.runtime.BoxedUnit;
  method func of type (): Unit has incompatible type
  override def func(): Unit = ???

If change Unit into BoxedUnit, like this:

class A1 extends A0 {
  override def func(): BoxedUnit= ???
}

another error occurs:

A0.scala:9:16
error overriding method func in trait I0 of type (): Unit;
  method func of type (): scala.runtime.BoxedUnit has incompatible type
  override def func(): BoxedUnit = ???

Expectation

The above code should compile successfully.

[sjrd]

BoxedUnit is an implementation detail. If you directly refer to it, you're on your own, and it's not the fault of the compiler if something doesn't work.

[XYZboom]

BoxedUnit is an implementation detail. If you directly refer to it, you're on your own, and it's not the fault of the compiler if something doesn't work.

Oh, I see, so what should I do if I expect to override the Scala method I1.func in Java

[XYZboom]

@sjrd If I use void to override I1.func in Java in A0, the Java compiler told me void is incompatible with BoxedUnit, so I have to use BoxedUnit (Unit is also incompatible in Java). But the Scala compiler does not recognize BoxedUnit

[sjrd]

I don't think you can do that in any meaningful way. Unit is just not first-class in Java, like it is in Scala.

[XYZboom]

I don't think you can do that in any meaningful way. Unit is just not first-class in Java, like it is in Scala.

Yes, Unit is not first-class in Java. But sometimes certain specific classes must be written in Java to compile, such as the following example:

interface I0 {
    public default void func() {}
}
public class A0  {
    public final void func() {}
}
class A1 extends A0 implements I0 {}

But the same scala code does not compile:

trait I0 {
  def func(): Unit = {}
}
open class A0 {
  final def func(): Unit = {}
}
class A1 extends A0, I0 {
}

Error:

A0.scala:7:7
error overriding method func in class A0 of type (): Unit;
  method func in trait I0 of type (): Unit cannot override final member method func in class A0
class A1 extends A0, I0 {

I think this is because Scala does not allow final methods in classes to directly override the default implementation in traits.
So when such a situation occurs, I need a Java class. Or rather, when certain other situations occur, I must use a Java class. Or, to put it another way, someone may have a project that was previously written entirely in Java, and want to gradually migrate to Scala. In short, we must use a Java class instead of a Scala class in certain situations, which may be the intended usage scenario of this issue.