Override check is optional for contravariant traits

[joroKr21]

Clone of scala/bug#12209 - I thought it's worth reporting here as well because it looks like a problematic interaction of language features: overriding + overloading + traits + variance.

Minimized code

object OverrideCheck extends App {
  trait A { def m(x: CharSequence): String = s"A: $x" }
  //trait B extends A { override def m(x: String): String = s"B: $x" } // no: m has a different signature

  trait C[-T <: CharSequence] { def m(x: T): String = s"C: $x" }
  //trait D extends C[String] { def m(x: String): String = "D" } // no: m needs override modifier
  trait D extends C[String] { override def m(x: String): String = s"D: $x" } // ok

  val x = new D with C[CharSequence] // ok: String <: CharSequence, C[CharSequence] <: C[String]
  println(x.m("x1")) // ok: D
  println((x: C[String]).m("x2")) // bad: C

  val y = new D {}
  println(y.m("y1")) // ok: D
  println((y: C[String]).m("y2")) // ok: D
}

I'm not sure if it's possible to observe this problem with a covariant trait.

Output

D: x1
C: x2
D: y1
D: y2

Expectation

I am not sure. I guess I shouldn't be able to define x.

I guess the problem is that we do the override check during method declaration, but the example shows that we can't know for sure at this point of time. Is it viable to do the override check for all members during class definition?