Make Eq a derivable typeclass

Author: odersky

library/src/scala/Eq.scala

@@ -12,6 +12,7 @@ sealed trait Eq[-L, -R]
  *  any instance of `Eq`.
  */
 object Eq extends Eq[Any, Any] {
+  def derived: Eq[Any, Any] = Eq
 
   /** A fall-back "implicit" to compare values of any types.
    *  Even though this method is not declared implicit, the compiler will

tests/neg/derive-eq.scala

@@ -0,0 +1,31 @@
+
+case class One() derives Eq
+case class Two() derives Eq
+
+implied for Eq[One, Two] = Eq
+
+enum Lst[T] derives Eq {
+  case Cons(x: T, xs: Lst[T])
+  case Nil()
+}
+
+case class Triple[S, T, U] derives Eq
+
+
+object Test extends App {
+  Test1
+  implicitly[Eq[Lst[Lst[One]], Lst[Lst[Two]]]]
+  implicitly[Eq[Triple[One, One, One],
+                Triple[Two, Two, Two]]]
+
+  val x: Triple[List[One], One, Two] = ???
+  val y: Triple[List[Two], One, Two] = ???
+  val z: Triple[One, List[Two], One] = ???
+  x == y // OK
+  x == x // OK
+  y == y // OK
+
+  y == x // error
+  x == z // error
+  z == y // error
+}

tests/neg/equality.scala

@@ -64,9 +64,7 @@ object equality {
     1 == null    // OK by eqIntNum
 
 
-    class Fruit
-
-    implicit def eqFruit: Eq[Fruit, Fruit] = Eq
+    class Fruit derives Eq
 
     class Apple extends Fruit
     class Pear extends Fruit

tests/run/derive-eq.scala

@@ -0,0 +1,23 @@
+
+case class One() derives Eq
+case class Two() derives Eq
+
+implied for Eq[One, Two] = Eq
+
+enum Lst[T] derives Eq {
+  case Cons(x: T, xs: Lst[T])
+  case Nil()
+}
+
+class Triple[S, T, U] derives Eq
+
+
+object Test extends App {
+  implicitly[Eq[Lst[Lst[One]], Lst[Lst[Two]]]]
+  implicitly[Eq[Triple[One, One, One],
+                Triple[Two, Two, Two]]]
+
+  val x: Triple[List[One], One, Two] = ???
+  val y: Triple[List[Two], One, Two] = ???
+  assert(x == y)
+}