Cannot extend some Scala abstract classes from Java

[hamzaremmal]

Compiler version

dfacd3f

Minimized code

Scala File:

class Box[T]

trait Foo:
    def foo: Box[this.type] = ???

abstract class Bar extends Foo:
    override def foo: Box[this.type] = ???

Java file:

class JBar extends Bar { }

Output

Javac will complain.

test.java:1: error: foo() in Bar cannot implement foo() in Foo
class JBar extends Bar { }
^
  return type Box<Bar> is not compatible with Box<Foo>
1 error

Expectation

To work. Although, this is up to debate.

Note

Changing the trait to an abstract class works. I believe this is actually a bug in the JVM given that this is against the specification.

[hamzaremmal]

I can also reproduce the same error in Java if the Scala file changes to:

class Box[+T]

trait Foo:
    def foo: Box[Foo] = ???

abstract class Bar extends Foo:
    override def foo: Box[Bar] = ???

[hamzaremmal]

The main issue is that when we generate the byte code, we also generate generic signature which we would have to approximate because the Java type system is not as powerful as Scala's. Now, the scope of this issue could be seen as is there room for improvements for these approximations or no.

[hamzaremmal]

Also, making foo in trait Foo abstract has its own can of worms but it is a direct result of the approximations of the generic signatures.

test.java:2: error: JBar is not abstract and does not override abstract method foo() in Foo
class JBar extends Bar { }
^
1 error

[hamzaremmal]

Close to this issue: #13104