Allow case objects to derive CanEqual and be compared in match statements.

[felix-hedenstrom]

I noticed this behavior when I started working with strictEquality and had my hopes that it would be addressed during
SIP-67, even if it is not explicitly mentioned I feel like it is a very similar issue. I'm using CanEqual and strictEquality at work in a way very similar to this

//> using scala 3.8.1
//> using option -language:strictEquality

case object Foo derives CanEqual


object Test {
  val x: Long | Foo.type = Foo

  val _ = x match {
    case l: Long => println("very long")
    case Foo => println("Very foo")
  }
}

which fails with

[error] ./example.sc:12:10
[error] Values of types object example$_.this.Foo and Long | example$_.this.Foo.type cannot be compared with == or !=
[error]     case Foo => println("Very foo")
[error]          ^^^

It compiles if I write

  val _ = x match {
    case l: Long => println("very long")
    case _: Foo.type => println("Very foo")
  }

but subjectively this is uglier and not how we used to write code before we started working with strictEquality.

[felix-hedenstrom]

Note that the original code compiles if there is a given CanEqual[Long | Foo.type, Long | Foo.type] = CanEqual.derived present.

[bishabosha]

so the issue is really that strict equality is not compatible with CanEqual - i.e. perhaps it could be automatically derived for union types, as user might expect that if a part of a union type has a CanEqual then it could be compatible