Stratify classTags in DottyPredef

Test case in pending/run/array-addition.scala. The problem was that
implicit search failed to insert `genericArrayOps` around `Array()`.
The reason was that the implicit parameter for `genericArrayOps` was
ambiguous, multiple `classTag` methods matched the expected type
`ClassTag[?T >: Nothing]`.

Stratifying ClassTags in DottyPredef into layers according
to implicit priority avoids this problem. This is a temporary solution
until we have proper ClassTag handling in place.

Author: odersky

src/dotty/DottyPredef.scala

@@ -4,22 +4,34 @@ import scala.reflect.ClassTag
 import scala.reflect.runtime.universe.TypeTag
 import scala.Predef.???  // this is currently ineffective, because of #530
 
-object DottyPredef {
-  /** implicits for ClassTag and TypeTag. Should be implemented with macros */
+abstract class I1 {
   implicit def classTag[T]: ClassTag[T] = scala.Predef.???
   implicit def typeTag[T]: TypeTag[T] = scala.Predef.???
-
-
-  /** ClassTags for final classes */
+  implicit val DoubleClassTag: ClassTag[Double]   = ClassTag.Double
+}
+abstract class I2 extends I1 {
+  implicit val FloatClassTag: ClassTag[Double]   = ClassTag.Double
+}
+abstract class I3 extends I2 {
+  implicit val LongClassTag: ClassTag[Long]       = ClassTag.Long
+}
+abstract class I4 extends I3 {
   implicit val IntClassTag: ClassTag[Int]         = ClassTag.Int
-  implicit val ByteClassTag: ClassTag[Byte]       = ClassTag.Byte
+}
+abstract class I5 extends I4 {
   implicit val ShortClassTag: ClassTag[Short]     = ClassTag.Short
+}
+abstract class I6 extends I5 {
+  implicit val ByteClassTag: ClassTag[Byte]       = ClassTag.Byte
   implicit val CharClassTag: ClassTag[Char]       = ClassTag.Char
-  implicit val LongClassTag: ClassTag[Long]       = ClassTag.Long
-  implicit val FloatClassTag: ClassTag[Float]     = ClassTag.Float
-  implicit val DoubleClassTag: ClassTag[Double]   = ClassTag.Double
   implicit val BooleanClassTag: ClassTag[Boolean] = ClassTag.Boolean
   implicit val UnitClassTag: ClassTag[Unit]       = ClassTag.Unit
   implicit val NullClassTag: ClassTag[Null]       = ClassTag.Null
+}
+
+/** implicits for ClassTag and TypeTag. Should be implemented with macros */
+object DottyPredef extends I6 {
+
+  /** ClassTags for final classes */
   implicit val NothingClassTag: ClassTag[Nothing] = ClassTag.Nothing
 }