Unexpected compilation error while calling Scala from Java

[anthony-cros]

Reproduction steps

Scala version: 2.13.8 (also in 2.12.x)

// $ cat Bar.scala:

trait Bar[F] {
          def f  : F
    final def bar: F = f // the real life example is more involved here
}

// ==================================================
//$ cat Foo.scala:

class Foo() extends Bar[Foo] {
    override def f: Foo = this
}


// ==================================================
//$ cat Baz.java:

class Baz {
    public static void main(String args[]) {
    //  Foo foo =       new Foo().bar();  // compilation error: incompatible types: Object cannot be converted to Foo
        Foo foo = (Foo)(new Foo().bar()); // compiles
    }
}

Problem

See more details on Scala Users: https://users.scala-lang.org/t/issue-with-calling-scala-code-from-java/8321

[jxnu-liguobin]

java15 and scala2.13.x also find error.
And it's interesting, it has been converted implicitly in scala.

object Test {
  // $ cat Bar.scala:

  trait Bar[F] {
    def f  : F
    final def bar: F = f // the real life example is more involved here
  }

  // ==================================================
  //$ cat Foo.scala:

  class Foo() extends Bar[Foo] {
    override def f: Foo = this
  }

  val useInScala =  new Foo().bar
}

show-trees, There have a asInstanceOf for converting as Foo type

    <stable> <accessor> def useInScala(): sandbox.Test.Foo = Test.this.useInScala;
    def <init>(): sandbox.Test.type = {
      Test.super.<init>();
      Test.this.useInScala = new sandbox.Test.Foo().bar().$asInstanceOf[sandbox.Test.Foo]();
      ()
    }

[Jasper-M]

I think this is #8905.

[jxnu-liguobin]

I think this is #8905.

So it seems impossible to solve?

[SethTisue]

Closing as duplicate. If someone is interested in reviving efforts to address #8905, let's discuss that there rather than here.