trait encoding introduces variance error

[scabug]

Clearly there is a bug here, but I don't know whether the bug is that both should compile or that neither should. If protected[this] is supposed to turn off variance checks like private[this] does, I don't see it in the spec. So I'm supposing neither should compile.

The following fails with a variance error:

trait C1[+A, +CC[X]] {
  protected[this] def f: A => CC[A] = sys.error("")
}

trait C2[+A, +CC[X]] extends C1[A, CC] {
  override protected[this] def f = super.f
}

// bug.scala:6: error: covariant type A occurs in contravariant position in type => (A) => CC[A] of method super$$f
//   override protected[this] def f = super.f
//                                          ^
// one error found

If we change C2 to a class, it compiles.

trait C1[+A, +CC[X]] {
  protected[this] def f: A => CC[A] = sys.error("")
}

class C2[+A, +CC[X]] extends C1[A, CC] {
  override protected[this] def f = super.f
}

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-4345?orig=1
Reporter: @paulp

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@ingoem said:
Maybe related to #3272.

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@adriaanm said:
(In r24539) closes #4345. skip variance checks for calls to super accessor. no review