method cannot override immutable value

[scabug]

The following code compiles with 2.12 (but it shouldn't) and doesn't compile with plain 2.11

abstract class Node(name: String) {
  var value: Option[Boolean] = None
}

case class Group(name: String, nodes: Node*) extends Node(name) {
  override def value = ???

  override def value_=(b: Option[Boolean]) = ???

}

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-9840?orig=1
Reporter: Svyatoslav Ilinskiy (ilinum)
Affected Versions: 2.12.0-M4

[scabug]

@soc said:
Hi Svyatoslav,

maybe I'm not getting the title of this bug report ... what did you have in mind when you wrote "immutable"?

[scabug]

Svyatoslav Ilinskiy (ilinum) said:
Hi Simon,

That's the error message you get in 2.11 when trying to compile the code.
Feel free to change the title if it makes it easier for people to understand

[scabug]

@adriaanm said:
Fails as follows in 2.12.0-{M4,M5} for me. Expected to have a different failure mode when compiling under -Xexperimental due to -Yoverride-vars having been included (until recently).

/Users/adriaan/Library/Preferences/IdeaIC2016.2/scratches/scratch_7.scala:6: error: overriding variable value in class Node of type Option[Boolean];
 method value cannot override a mutable variable
  override def value = ???
               ^
/Users/adriaan/Library/Preferences/IdeaIC2016.2/scratches/scratch_7.scala:8: error: overriding variable value in class Node of type Option[Boolean];
 method value_= cannot override a mutable variable
  override def value_=(b: Option[Boolean]) = ???
               ^
two errors found

[scabug]

Svyatoslav Ilinskiy (ilinum) said:
What do you mean by different failure mode? The code compiles for me with -Xexperimental and 2.11.8. Is this the case for you?

Tried it again with 2-12-M5, it does not compile indeed.