Confusing behaviour with abstract var and override

[oyvindberg]

Compiler version

3.0.0

Minimized code

// no `override` - fine
trait Ok1 { var i: Int }
class Ok1C extends Ok1 { var i: Int = 1 } 

// `override` of desugared `var` - fine
trait Ok2 {
  def i: Int
  def i_=(v: Int): Unit
}
class Ok2C extends Ok2 { override var i: Int = 1 }

// fails
trait NotOk {var i: Int}
class NotOkC extends NotOk { override var i: Int = 1 }

Output

error overriding variable i in trait NotOk of type Int;
  variable i of type Int cannot override a mutable variable

Expectation

override is used to implement abstract or re-implement concrete things, and guarantees their presence. In this case it seems impossible to use override to implement an abstract var.

The example compiles in scala 2

[som-snytt]

I submitted a quickie PR. override of abstract var is fine.

[odersky]

The fix in #13744 was not semantically correct.

When did Scala-2's behavior change? I tried with 2.13.5 and it also flags overrides of variables as errors.

[smarter]

When did Scala-2's behavior change?

In scala/scala@9b59f5f which is part of 2.12.0. I tried it in both 2.12.0 and 2.13.5 and could get the following code to compile:

trait NotOk { var i: Int }
class NotOkC extends NotOk { override var i: Int = 1 }