Add Product{1,2} supertrait to case classes

Case classes with arity <= 1 now also get a ProductN parent trait.
This is necessary because we inherit productArity and Element methods
in case classes from the ProdictN trait.

Needed to add Product0 for this, which is not defined in Scala2.x.

Author: odersky

src/dotty/tools/dotc/ast/Desugar.scala

@@ -312,12 +312,12 @@ object desugar {
     def productConstr(n: Int) = {
       val tycon = ref(defn.ProductNClass(n).typeRef)
       val targs = constrVparamss.head map (_.tpt)
-      AppliedTypeTree(tycon, targs)
+      if (targs.isEmpty) tycon else AppliedTypeTree(tycon, targs)
     }
 
     // Case classes get a ProductN parent
     var parents1 = parents
-    if ((mods is Case) && 2 <= arity && arity <= Definitions.MaxTupleArity)
+    if ((mods is Case) && arity <= Definitions.MaxTupleArity)
       parents1 = parents1 :+ productConstr(arity)
 
     // The thicket which is the desugared version of the companion object

src/dotty/tools/dotc/core/Definitions.scala

@@ -394,7 +394,7 @@ class Definitions {
     lazy val Function0_apply = FunctionClass(0).requiredMethod(nme.apply)
 
   lazy val TupleClass = mkArityArray("scala.Tuple", MaxTupleArity, 2)
-  lazy val ProductNClass = mkArityArray("scala.Product", MaxTupleArity, 2)
+  lazy val ProductNClass = mkArityArray("scala.Product", MaxTupleArity, 0)
 
   lazy val FunctionClasses: Set[Symbol] = FunctionClass.toSet
   lazy val TupleClasses: Set[Symbol] = TupleClass.toSet

src/scala/Product0.scala

@@ -0,0 +1,24 @@
+/*                     __                                               *\
+**     ________ ___   / /  ___     Scala API                            **
+**    / __/ __// _ | / /  / _ |    (c) 2002-2013, LAMP/EPFL             **
+**  __\ \/ /__/ __ |/ /__/ __ |    http://scala-lang.org/               **
+** /____/\___/_/ |_/____/_/ | |                                         **
+**                          |/                                          **
+\*                                                                      */
+package scala
+
+/** A class for Product0 which was missing from the scala distribution. */
+object Product0 {
+  def unapply(x: Product0): Option[Product0] =
+    Some(x)
+}
+
+trait Product0 extends Any with Product {
+
+  override def productArity = 0
+
+  @throws(classOf[IndexOutOfBoundsException])
+  override def productElement(n: Int) =
+    throw new IndexOutOfBoundsException(n.toString())
+}
+