Cannot override var setters in sub-class of sub-class - "error overriding variable … cannot override a mutable variable"

[Sciss]

reproduction steps

using Scala 3.0.2,

trait Foo

trait Top:
  var foo: Foo

trait Middle extends Top:
  def foo: Foo = ???
  def foo_=(value: Foo): Unit = ???

class Bottom extends Middle:
  // error overriding variable foo in trait Top of type Foo;
  // method foo_= of type (value: Foo): Unit cannot override a mutable variable
  override def foo_=(value: Foo): Unit =
    super.foo_=(value)
    println("Helo")

If I rewrite Top as

trait Top:
  def foo: Foo
  def foo_=(value: Foo): Unit

"it works"

problem

I think it should always be possible to override getters and setters for a var.

[SethTisue]

please report Scala 3 bugs at https://github.com/lampepfl/dotty/issues

[Sciss]

sorry, I keep forgetting that the bug trackers are still separate

[SethTisue]

no worries 😀

scala/scala3#13738

[som-snytt]

Duplicates #9840 (Edit: I read it wrong)

That would be a dotty feature-request.

[Sciss]

I don't understand why it is a feature request. This works fine in Scala 2, and it is not comprehensible why if I define the Top trait interface, I have to go back to change the interface depending on some forms of later implementations. This doesn't make sense. I have just changed in one library

trait Scope {
   var logFreq: Boolean

   var logFreqMin: Double

   ... etc

to

trait Scope {
   def logFreq: Boolean
   def logFreq_=(value: Boolean): Unit

   def logFreqMin: Double
   def logFreqMin_(value: Double): Unit

   ... etc

purely defensively and in order to later implement that thing. I shouldn't need to change API definitions based on how the implementations (perhaps even in other libraries) are going. The duplicate issue you link to is very old imported from 2016 bug tracker and closed without resolution.

Voting to re-open this issue.

[som-snytt]

IDK offhand, but I assumed it has to do with whether a field is allocated. Just analyzing aliased member fields is hard, but impossible with overridden accessors? Just guessing, and I'm not an expert.

[dwijnand]

Voting to re-open this issue.

The issue is open (in the right tracker): scala/scala3#13738

[Sciss]

The issue is open

🤦 I'll get this eventually

[som-snytt]

I was saying it's the wrong tracker, it should be a feature request. :)

[som-snytt]

Sorry, somehow I missed that the var is abstract here. I fixed the dotty issue under a different ticket after 3.1.0.