Test case for multiversal equality

Author: odersky

tests/neg/equality.scala

@@ -0,0 +1,61 @@
+import dotty.{Eq,EqClass}
+
+object equality {
+
+  case class Str(str: String) extends EqClass[_]
+
+  case class Num(x: Int) extends EqClass[Num]
+
+  case class Other(x: Int)
+
+  trait Option[+T] extends EqClass[_]
+  case class Some[+T](x: T) extends Option[T]
+  case object None extends Option[Nothing]
+
+  implicit def eqStr: Eq[Str] = Eq
+  implicit def eqOption[T: Eq]: Eq[Option[T]] = Eq
+
+  def main(args: Array[String]): Unit = {
+    val x = Str("abc")
+    x == x
+
+    val n = Num(2)
+    val m = Num(3)
+    n == m
+
+    Other(1) == Other(2)
+
+    Some(x) == None
+    Some(x) == Some(Str(""))
+    val z: Option[Str] = Some(Str("abc"))
+    z == Some(x)
+    z == None
+    Some(x) == z
+    None == z
+
+    Other(3) == null
+    Str("x") == null
+    null == Other(3)
+    null == Str("x")
+    null == null
+
+    def ddistinct[T: Eq](xs: List[T]): List[T] = xs match {
+      case Nil => Nil
+      case x :: xs => x :: xs.filterNot(x == _)
+    }
+
+    ddistinct(List(z, z, z))
+
+    x == n  // error
+    n == x  // error
+    x == Other(1)  // error
+    Other(2) == x // error
+    z == Some(n) // error
+    z == n // error
+    Some(n) == z // error
+    n == z // error
+    Other(1) == z // error
+    z == Other(1) // error
+    ddistinct(List(z, n)) // error
+  }
+}