Missing bridge method in trait when it has a covariant return type

[XYZboom]

Compiler version

3.6.3-RC2

Minimized code

trait I0 {
  def func(): AnyRef
}

trait I1 extends I0 {
  override def func(): String = "123"
}

Output

Use javap -c -s- v I1 to check the bytecode: (Simplified it a bit)

  public static java.lang.String func$(I1);
    descriptor: (LI1;)Ljava/lang/String;
    flags: ACC_PUBLIC, ACC_STATIC

  public java.lang.String func();
    descriptor: ()Ljava/lang/String;
    flags: ACC_PUBLIC

Expectation

When I1 is written by Java, the output is:

  public java.lang.String func();
    descriptor: ()Ljava/lang/String;
    flags: ACC_PUBLIC

  public java.lang.Object func();
    descriptor: ()Ljava/lang/Object;
    flags: ACC_PUBLIC, ACC_BRIDGE, ACC_SYNTHETIC
}

Note that java does not generate static method func$ here. But scalac misses a bridge method here.

Further bug: java.lang.AbstractMethodError when implements I1 with a Java class

trait I0 {
  def func(): AnyRef
}

trait I1 extends I0 {
  override def func(): String = "123"
}
object Main {
  def main(args: Array[String]): Unit = {
    val i0: I0 = A()
    println(i0.func())
  }
}

// Java file
public class A implements I1 {
}

Exception in thread "main" java.lang.AbstractMethodError: Method A.func()Ljava/lang/Object; is abstract
	at A.func(A.java)
	at Main$.main(A1.scala:14)
	at Main.main(A1.scala)

[hamzaremmal]

According to https://docs.scala-lang.org/scala3/book/interacting-with-java.html#how-to-use-scala-traits-in-java, it should work 😄. The behavior was introduced in #2342 and the implementation behaves exactly as documented in that PR (which should change).