Straw man for multiversal equality.

This test shows how we can make equality non-universal in Scala. It also
exhibited the two bugs fixed in the previous two commits.

Also related: SI-9763.

Author: odersky

tests/neg/EqualityStrawman1.scala

@@ -0,0 +1,76 @@
+package strawman.equality
+
+object EqualityStrawman1 {
+
+  trait Eq[-T]
+  trait Impossible
+
+  object Eq extends Eq[Any]
+
+  trait Base {
+    def === (other: Any): Boolean = this.equals(other)
+    def === (other: CondEquals)(implicit ce: Impossible): Boolean = ???
+  }
+
+  trait CondEquals extends Base {
+    def === [T >: this.type <: CondEquals](other: T)(implicit ce: Eq[T]): Boolean = this.equals(other)
+    def === (other: Any)(implicit ce: Impossible): Boolean = ???
+  }
+
+  trait Equals[-T] extends CondEquals
+
+  case class Str(str: String) extends CondEquals
+
+  case class Num(x: Int) extends Equals[Num]
+
+  case class Other(x: Int) extends Base
+
+  trait Option[+T] extends CondEquals
+  case class Some[+T](x: T) extends Option[T]
+  case object None extends Option[Nothing]
+
+  implicit def eqStr: Eq[Str] = Eq
+  //implicit def eqNum: Eq[Num] = Eq
+  implicit def eqOption[T: Eq]: Eq[Option[T]] = Eq
+
+  implicit def eqEq[T <: Equals[T]]: Eq[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
+
+
+    def ddistinct[T <: Base: 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
+  }
+}