Compiler version

3.6.3-RC2, 3.6.4-RC1-bin-20241231-1f0c576-NIGHTLY

Minimized code

// FILE: I0.java public interface I0<T> { public default T func(T t) { return t; } } // FILE: I1.java public interface I1 extends I0<String> { @Override default String func(String s) { return s; } }

abstract class A0 extends I1 with I0[Object] { override def func(s: Object): String = super.func(s) }

Output

passed

Expectation

error: class A0 has conflicting base types I0[String] and I0[Object].
We can see a similar but not very clear error in scala2:

illegal inheritance; self-type A0 does not conform to I0[Object]'s selftype I0[Object]