Add new strawman for multiversal equality

tests/neg/EqualityStrawman2.scala

@@ -0,0 +1,96 @@
+object equality {
+
+  trait Eq[-T, -U]
+  object Eq extends Eq[Any, Any]
+
+  implicit class EqualsDeco[T](val x: T) extends AnyVal {
+    def ===[U] (y: U)(implicit ce: Eq[T, U]) = x.equals(y)
+  }
+
+  type EqEq[T] = Eq[T, T]
+
+  trait EqClass
+
+  implicit def eqAny: Eq[Any, Any] = Eq
+
+  implicit def mixedEq1   : Eq[Any, EqClass] = Eq
+  implicit def mixedEq1alt: Eq[Any, EqClass] = Eq
+  implicit def mixedEq2   : Eq[EqClass, Any] = Eq
+  implicit def mixedEq2alt: Eq[EqClass, Any] = Eq
+
+//  implicit def eqEq[U, T <: U with EqClass[U]]: Eq[T, T] = Eq
+
+  case class Str(str: String) extends EqClass
+  case class Num(x: Int) extends EqClass
+
+  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, Str] = Eq
+  implicit def eqNum: Eq[Num, Num] = Eq
+  implicit def eqOption[T, U](implicit ce: Eq[T, U]): Eq[Option[T], Option[U]] = Eq
+
+  def main(args: Array[String]): Unit = {
+    val x = Str("abc")
+    (Some(x): Option[Str]) === (None: Option[Str])
+
+    x === x
+
+    val n = Num(2)
+    val m = Num(3)
+    n === m
+
+    Other(1) === Other(2)
+
+    (Some(x) === None)//(eqOption)
+    (Some(x) === Some(Str("")))(eqOption)
+    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*/
+
+    class Fruit extends EqClass
+    class Apple extends Fruit
+    class Pear extends Fruit
+    implicit def eqFruit: Eq[Fruit, Fruit] = Eq
+
+    Some(new Apple) === Some(new Pear)
+
+
+    def ddistinct[T: EqEq](xs: List[T]): List[T] = xs match {
+      case Nil => Nil
+      case x :: xs => x :: xs.filterNot(x === _)
+    }
+
+    ddistinct(List(z, z, z))
+
+    n match {
+      case None =>
+    }
+
+    Some(new Apple) === Some(Str("xx")) // error
+    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
+
+
+  }
+}