Fix isUnboundedGeneric for &/| types

This also fixes #1747 by allowing the same encoding as `scalac` to cope
with overridden methods over unbounded generic arrays.

Author: odersky

compiler/src/dotty/tools/dotc/core/TypeErasure.scala

@@ -211,8 +211,8 @@ object TypeErasure {
     case tp: TypeAlias => isUnboundedGeneric(tp.alias)
     case tp: TypeBounds => !tp.hi.derivesFrom(defn.ObjectClass)
     case tp: TypeProxy => isUnboundedGeneric(tp.underlying)
-    case tp: AndType => isUnboundedGeneric(tp.tp1) || isUnboundedGeneric(tp.tp2)
-    case tp: OrType => isUnboundedGeneric(tp.tp1) && isUnboundedGeneric(tp.tp2)
+    case tp: AndType => isUnboundedGeneric(tp.tp1) && isUnboundedGeneric(tp.tp2)
+    case tp: OrType => isUnboundedGeneric(tp.tp1) || isUnboundedGeneric(tp.tp2)
     case _ => false
   }
 

tests/neg/i1747.scala

@@ -1,3 +1,3 @@
-class Coll[E] extends java.util.Collection[E] { // error: needs to be abstract
-  def toArray[T](a: Array[T]): Array[T] = ??? // error: cannot override
+abstract class Coll[E] extends java.util.Collection[E] {
+  override def toArray[T](a: Array[T]): Array[T] = ??? // error: has different signature
 }

tests/pos/i1747.scala

@@ -0,0 +1,3 @@
+abstract class Coll[E] extends java.util.Collection[E] {
+  override def toArray[T](a: Array[T with Object]): Array[T with Object] = ??? // error: cannot override
+}