strictEquality doesn't work with GADTs

[japgolly]

Minimized code

//> using scala 3.7.4
//> using option -language:strictEquality

sealed trait Op derives CanEqual
case object OpA extends Op

object Test1:
  val op: Op => Int =
    case OpA => 42

sealed trait OpX[X] derives CanEqual
case object OpXA extends OpX[Int]

object Test2:
  def op[A]: OpX[A] => Int =
    case OpXA => 42 // ❌

Output

[error] ./what.scala:16:10
[error] Values of types object OpXA and OpX[A] cannot be compared with == or !=.
[error] I found:
[error] 
[error]     OpX.derived$CanEqual[Int, A](/* missing */summon[CanEqual[Int, A]])
[error] 
[error] But no implicit values were found that match type CanEqual[Int, A]
[error] 
[error] where:    A is a type in method op which is an alias of Int
[error] .
[error]     case OpXA => 42 // ❌
[error]          ^^^^

Expectation

Successful compilation.

[eejbyfeldt]

I think the behavior here actually the expected behavior. The derives CanEqual will generate a given like

  given [XR, XL](using CanEqual[XR, XL]): CanEqual[OpX[XR], OpX[XL]] = CanEqual.derived

So the when the compiler tries to use this given it looks for CanEqual[Int, A].

Depending on how simplified the case is it might be improved by this SIP: scala/improvement-proposals#97

But for now I guess one could work around it by defining more CanEqual instances. Like

case object OpXA extends OpX[Int] {
  given [X]: CanEqual[OpXA.type, OpX[X]] = CanEqual.derived
}

or making the derived one less constrained

object OpX {
  given [XR, XL]: CanEqual[OpX[XR], OpX[XL]] = CanEqual.derived
}