conflicting instances with inheritance from case class

[SimonGuilloud]

Compiler version

Tested with 3.2.1 and 3.3.1

Minimized code

trait A
trait B extends A

trait M[+T]

case class MA(id:Int) extends M[A]
class MB(id:Int)  extends MA(id) with M[B]

Output

illegal inheritance: class MB inherits conflicting instances of non-variant base trait M.

  Direct basetype: M[Playground.B]
  Basetype via case class MA: M[A]

Expectation

Compiles, or meaningful error message. If MA is not a case class, the code compiles. The base trait M is not non-variant.

[soronpo]

Compiles successfully on 2.13

[SimonGuilloud]

I accidentally rediscovered the bug while forgetting it was reported already. Does someone have an idea of a workaround other than implementing manually all the functions of case classes?

[EugeneFlesselle]

@SimonGuilloud not sure if it would be suitable for your use case, but you can do:

trait M:
  type T

case class MA(id:Int) extends M:
  type T <: A
class MB(id:Int) extends MA(id):
  type T <: B