Add CanEqual typeclass instances of Option, Either and Tuple

- In addition to CanEqual[Option[T], Option[U]], CanEqual[Option[T], Option[T]] is also added for None case in pattern matching.
- CanEqual instances for Tuple are added to Tuple companion object since Tuple is Scala 3's new type unlike Option and Either.

library/src/scala/CanEqual.scala

@@ -31,4 +31,13 @@ object CanEqual {
   // source code of these classes
   given canEqualSeq[T, U](using eq: CanEqual[T, U]): CanEqual[Seq[T], Seq[U]] = derived
   given canEqualSet[T, U](using eq: CanEqual[T, U]): CanEqual[Set[T], Set[U]] = derived
+
+  // The next three typeclass instances can also go into the companion objects of classes Option and Either.
+  // For now they are here in order not to have to touch the source code of these classes
+  given canEqualOptions[T, U](using eq: CanEqual[T, U]): CanEqual[Option[T], Option[U]] = derived
+  given canEqualOption[T](using eq: CanEqual[T, T]): CanEqual[Option[T], Option[T]] = derived // for `case None` in pattern matching
+
+  given canEqualEither[L1, R1, L2, R2](
+    using eqL: CanEqual[L1, L2], eqR: CanEqual[R1, R2]
+  ): CanEqual[Either[L1, R1], Either[L2, R2]] = derived
 }

library/src/scala/Tuple.scala

@@ -232,6 +232,11 @@ object Tuple {
 
   def fromProductTyped[P <: Product](p: P)(using m: scala.deriving.Mirror.ProductOf[P]): m.MirroredElemTypes =
     runtime.Tuples.fromProduct(p).asInstanceOf[m.MirroredElemTypes]
+
+  given canEqualEmptyTuple: CanEqual[EmptyTuple, EmptyTuple] = CanEqual.derived
+  given canEqualTuple[H1, T1 <: Tuple, H2, T2 <: Tuple](
+    using eqHead: CanEqual[H1, H2], eqTail: CanEqual[T1, T2]
+  ): CanEqual[H1 *: T1, H2 *: T2] = CanEqual.derived
 }
 
 /** A tuple of 0 elements */

tests/neg/equality1.scala

@@ -3,4 +3,118 @@ object equality1 {
   class A
   class B
   new A == new B // error: cannot compare
+
+  case class Foo(n: Int) derives CanEqual
+
+  sealed trait Status derives CanEqual
+  object Status {
+    case class Active(since: Int) extends Status
+    case object Pending extends Status
+    case object Inactive extends Status
+  }
+
+  enum Color derives CanEqual {
+    case Red
+    case Green
+    case Blue
+  }
+
+  val option1a: Option[Int] = Some(1)
+  val option1b: Option[Int] = Some(1)
+  option1a == option1b
+
+  option1a match {
+    case Some(1) =>
+      println("1")
+    case Some(n) =>
+      println("Not 1")
+    case None => // This None case doesn't work without CanEqual.canEqualOption[T]
+      println("None")
+  }
+
+  1 == '1'
+  val option2a: Option[Int] = Some(1)
+  val option2b: Option[Char] = Some('1')
+  option2a == option2b
+
+  val option3a: Option[Foo] = Some(Foo(1))
+  val option3b: Option[Foo] = Some(Foo(1))
+  option3a == option3b
+
+  val option4a: Option[Status] = Some(Status.Active(2020))
+  val option4b: Option[Status] = Some(Status.Pending)
+  val option4c: Option[Status] = Some(Status.Inactive)
+  option4a == option4b
+  option4b == option4c
+
+  val option5a: Option[Color] = Some(Color.Red)
+  val option5b: Option[Color] = Some(Color.Green)
+  val option5c: Option[Color] = Some(Color.Blue)
+  option5a == option5b
+  option5b == option5c
+
+  val optionError1a: Option[Int] = Some(1)
+  val optionError1b: Option[String] = Some("1")
+  optionError1a == optionError1b // error: cannot compare
+
+  val optionError2a: Option[Char] = Some('a')
+  val optionError2b: Option[String] = Some("a")
+  optionError2a == optionError2b // error: cannot compare
+
+  val optionTuple1a: Option[(Int, String)] = Some((1, "OK"))
+  val optionTuple1b: Option[(Int, String)] = Some((1, "OK"))
+  optionTuple1a == optionTuple1b
+
+  'a' == 97
+  val optionTuple2a: Option[(Int, Char)] = Some((1, 'a'))
+  val optionTuple2b: Option[(Int, Int)] = Some((1, 97))
+  optionTuple2a == optionTuple2b
+
+  val optionTupleError1a: Option[(Int, String)] = Some((1, "OK"))
+  val optionTupleError1b: Option[(String, Int)] = Some(("OK", 1))
+  optionTupleError1a == optionTupleError1b // error: cannot compare
+
+  val eitherL1a: Either[String, Int] = Left("Error")
+  val eitherL1b: Either[String, Int] = Left("Error")
+  eitherL1a == eitherL1b
+
+  val eitherR1a: Either[String, Int] = Right(999)
+  val eitherR1b: Either[String, Int] = Right(999)
+  eitherR1a == eitherR1b
+
+  val eitherErrorL1a: Either[String, Int] = Left("Error")
+  val eitherErrorL1b: Either[Char, Int] = Left('E')
+  eitherErrorL1a == eitherErrorL1b // error: cannot compare
+
+  val eitherErrorR1a: Either[String, Int] = Right(999)
+  val eitherErrorR1b: Either[String, String] = Right("999")
+  eitherErrorR1a == eitherErrorR1b // error: cannot compare
+
+
+  val eitherTupleL1a: Either[(String, Long), (Int, Boolean)] = Left(("Error", 123L))
+  val eitherTupleL1b: Either[(String, Long), (Int, Boolean)] = Left(("Error", 123L))
+  eitherTupleL1a == eitherTupleL1b
+
+  val eitherTupleR1a: Either[(String, Long), (Int, Boolean)] = Right((999, true))
+  val eitherTupleR1b: Either[(String, Long), (Int, Boolean)] = Right((999, true))
+  eitherTupleR1a == eitherTupleR1b
+
+  val eitherTupleErrorL1a: Either[(String, Long), (Int, Boolean)] = Left(("Error", 123L))
+  val eitherTupleErrorL1b: Either[(Long, String), (Int, Boolean)] = Left((123L, "Error"))
+  eitherTupleErrorL1a == eitherTupleErrorL1b // error: cannot compare
+
+  val eitherTupleErrorR1a: Either[(String, Long), (Int, Boolean)] = Right((999, true))
+  val eitherTupleErrorR1b: Either[(String, Long), (Boolean, Int)] = Right((true, 999))
+  eitherTupleErrorR1a == eitherTupleErrorR1b // error: cannot compare
+
+  (1, "a") == (1, "a")
+  (1, "a", true) == (1, "a", true)
+  (1, "a", true, 't') == (1, "a", true, 't')
+  (1, "a", true, 't', 10L) == (1, "a", true, 't', 10L)
+
+  (1, "a") == (1, 'a') // error: cannot compare
+  (1, "a") == ("a", 1) // error: cannot compare
+  (1, "a") == (1, "a", true)  // error: cannot compare
+  (1, "a", true, 't', 10L) == (1, "a", 1.5D, 't', 10L) // error: cannot compare
+
 }