Strict equality CanEqual derivation fails for classes in which type argument is not used for equality

[SimY4]

Compiler version

Scala 3.1.1

Minimized code

import scala.language.strictEquality

case class MyClass[A](value: String)(val a: A) derives CanEqual

class Something {}

val a = MyClass[Something]("some")(new Something())
val b = MyClass[Something]("some")(new Something())

println(a == b)

Output

Values of types Playground.MyClass[Playground.Something] and Playground.MyClass[Playground.Something] cannot be compared with == or !=.
I found:

    Playground.MyClass.derived$CanEqual[Playground.Something, Playground.Something](
      /* missing */summon[CanEqual[Playground.Something, Playground.Something]]
    )

But no implicit values were found that match type CanEqual[Playground.Something, Playground.Something].

Expectation

Something is not part of case class universal equality check so it shouldn't be part of multiversal equality constraint either

[spamegg1]

I cannot reproduce it:

 ➜ scala
Welcome to Scala 3.1.1 (17.0.1, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.
                                                                                                                                       
scala> case class MyClass[A](value: String)(val a: A) derives CanEqual
// defined case class MyClass
                                                                                                                                       
scala> class Something {}
// defined class Something
                                                                                                                                       
scala> val a = MyClass[Something]("some")(new Something())
val a: MyClass[Something] = MyClass(some)
                                                                                                                                       
scala> val b = MyClass[Something]("some")(new Something())
val b: MyClass[Something] = MyClass(some)
                                                                                                                                       
scala> println(a == b)
true

[SimY4]

@spameggo you need to enable strict quality via:

import scala.language.strictEquality

[nicolasstucki]

It seems that the code above is correctly complaining as there is no instance of CanEqual[Something, Something] in scope. This implicit is required because derives CanEqual generates the following given instance

given derived$CanEqual[L, R](implicit x$0: CanEqual[L, R]): CanEqual[MyClass[L], MyClass[R]] = CanEqual.derived

Note that under strict equality the comparison new Something == new Something will also not work without the CanEqual[Something, Something]. To make this code compile you can add

given CanEqual[Something, Something] = CanEqual.derived

or derive it on Somthing

class Something derives CanEqual {}

[nicolasstucki]

@odersky should derive generate givens with implicit parameters or using parameters?

- given derived$CanEqual[L, R](implicit x$0: CanEqual[L, R]): CanEqual[MyClass[L], MyClass[R]] = CanEqual.derived
+ given derived$CanEqual[L, R](using x$0: CanEqual[L, R]): CanEqual[MyClass[L], MyClass[R]] = CanEqual.derived

I would have expected a using.

[SimY4]

@nicolasstucki @odersky I would argue that my understanding of strict equality feature was merely about making equality checks safer at compiler time, not to actually change the semantics of case class equality.

[SimY4]

To be fair, I was hoping that CanEqual for case classes would simply delegate to case class generated equality.

[bishabosha]

To be fair, I was hoping that CanEqual for case classes would simply delegate to case class generated equality.

It does, CanEqual has no methods to call, the == operation still delegates to equals, it just checks at compiletime if there exists the correct CanEqual in implicit scope