Semantic bug when override var with val and no way to extends from a trait that has a var for a case class

[pootow]

Compiler version

3.1.1

Minimized code

trait HasId(var id: String)

case class Entity(override val id: String) extends HasId(id)

val entity = Entity("0001")
entity.id = "0002"
println(entity.id)

Output

0001

Expectation

Either does not compile or output 0002.

My intention is that I do not want to introduce a redundant variable just for assigning to trait parameter.

trait HasId(var id: String)

case class Entity(var idJustForAssignment: String) extends HasId(idJustForAssignment)

val entity = Entity("id")
entity.id
entity.idJustForAssignment // this is a duplication

and this does not work:

trait HasId(var id: String)

case class Entity(id: String) extends HasId(id) // and this works when it's a normal class not a case class

and this does not work either:

trait HasId(var id: String)

case class Entity(override var id: String) extends HasId(id)

So there's no way to extends from a trait that has a var for a case class.

[odersky]

The erroneous behavior was introduced in #13744. Before it was a compile error.

We need to make it a compile error again unless we can fix the underlying issues. This means jumping into trait_setter logic, which is not fun.

Meanwhile, the following will work:

trait HasId:
  def id: String
  def id_=(s: String): Unit

case class Entity(var id: String) extends HasId