Unsoundness when inheriting conflicting type parameters

[odersky]

The following program compiles under 2.12.3 but gives a ClassCastException at runtime.

import java.util.Comparator

trait Trait1[T] { def foo(arg: Comparator[T]): String }

trait Trait2[T] extends Trait1[T] { def foo(arg: Comparator[String]): Int = 0 }

class Class1 extends Trait2[String] { }

object Test {
  def main(args: Array[String]): Unit = {
    val c = new Class1
    c.foo(Ordering[String])
    val t: Trait1[String] = c
    val x: String = t.foo(Ordering[String])
  }
}

This is an adaptation of the example in Scala SI 4731.scala. The issue is in response to scala/scala3#3989. The fix for #3989 broke the test case pos/t4731.scala, and investigation showed that the flagged inheritance is unsound.

[Atry]

Currently I created some conflicting refinement type intentionally, in order to mimic the type variance behavior of intersection types.

Stricter type checking on conflicting types may be sound, however it will break existing code.

I hope this bug will not be fixed until introducing true intersection types.