(should forward-port/report to Scala 3 once approved)

👍🏽 for the canEqual approach.

@lrytz do we need to release-note this, might it cause mysterious breakage?

Yes, it changes case class equality.

scala> case class C(x: Int)
class C

scala> class D extends C(1) { override def productPrefix = "D" }
class D

scala> C(1) == new D
val res0: Boolean = false

That is true in 2.13.15.

we'll need a comparable PR on the Scala 3 side, yes?

Yes. I can submit it there to probe what the 3 folks think about the semantic change.

@lrytz where does this stand, after the discussion on scala/scala3#21606 ?

@sjrd points out that this fix goes the wrong way around; what we want is to define semantics of equals and then adjust the hashCode to be compatible. This PR does the opposite.

So here's my attempt to fix hashCode instead: #11023