pat-mat - Recursion limit exceeded in sub-type matching

[Sciss]

Minimized code

trait Txn[T <: Txn[T]]

trait Elem[T <: Txn[T]]

trait Obj[T <: Txn[T]] extends Elem[T]

trait Copy[In <: Txn[In], Out <: Txn[Out]] {
  def copyImpl[Repr[~ <: Txn[~]] <: Elem[~]](in: Repr[In]): Repr[Out]
  
  def apply[Repr[~ <: Txn[~]] <: Elem[~]](in: Repr[In]): Repr[Out] = {
    val out = copyImpl[Repr](in)
    (in, out) match {
      case (inObj: Obj[In], outObj: Obj[Out]) =>     // problem here
        println("copy the attributes")
      case _ =>
    }
    out
  }
}

Output

https://scastie.scala-lang.org/hzmyXFSrRoq4Na9ZMsIkTQ

Recursion limit exceeded.
Maybe there is an illegal cyclic reference?
If that's not the case, you could also try to increase the stacksize using the -Xss JVM option.
A recurring operation is (inner to outer):

  subtype de.sciss.lucre.experiment.Txn[
  LazyRef(
    de.sciss.lucre.experiment.Txn[
      LazyRef(
        de.sciss.lucre.experiment.Txn[
          LazyRef(
            de.sciss.lucre.experiment.Txn[
              LazyRef(
                de.sciss.lucre.experiment.Txn[
                  LazyRef(
                    de.sciss.lucre.experiment.Txn[
                      LazyRef(
                        de.sciss.lucre.experiment.Txn[
                          LazyRef(
                            de.sciss.lucre.experiment.Txn[
                              LazyRef(
                                de.sciss.lucre.experiment.Txn[
                                  LazyRef(
                                    de.sciss.lucre.experiment.Txn[
                                      LazyRef(
                                        de.sciss.lucre.experiment.Txn[
                                          LazyRef(...)
                                        ]
                                      )
                                    ]
                                  )
                                ]
                              )
                            ]
                          )
                        ]
                      )
                    ]
                  )
                ]
              )
            ]
          )
        ]
      )
    ]
  )
] <:< de.sciss.lucre.experiment.Txn[LazyRef(T)]
  subtype LazyRef(T) <:< LazyRef(
  de.sciss.lucre.experiment.Txn[
    LazyRef(
      de.sciss.lucre.experiment.Txn[
        LazyRef(
          de.sciss.lucre.experiment.Txn[
            LazyRef(
              de.sciss.lucre.experiment.Txn[
                LazyRef(
                  de.sciss.lucre.experiment.Txn[
                    LazyRef(
                      de.sciss.lucre.experiment.Txn[
                        LazyRef(
                          de.sciss.lucre.experiment.Txn[
                            LazyRef(
                              de.sciss.lucre.experiment.Txn[
                                LazyRef(
                                  de.sciss.lucre.experiment.Txn[
                                    LazyRef(
                                      de.sciss.lucre.experiment.Txn[
                                        LazyRef(
                                          de.sciss.lucre.experiment.Txn[
                                            LazyRef(...)
                                          ]
                                        )
                                      ]
                                    )
                                  ]
                                )
                              ]
                            )
                          ]
                        )
                      ]
                    )
                  ]
                )
              ]
            )
          ]
        )
      ]
    )
  ]
)
  subtype LazyRef(T) <:< LazyRef(
  de.sciss.lucre.experiment.Txn[
...

Expectation

should compile ( https://scastie.scala-lang.org/UL2Je3yATT6MukbzvP0qRA )

[Sciss]

Same happens when I replace pat-mat with if (in.isInstanceOf[Obj[In]] && out.isInstanceOf[Obj[Out]]) ; if I put place-holders like in.isInstanceOf[Obj[_]] and cast afterwards, it compiles. So doesn't actually require pattern matching.