Fix isGround logic

Author: odersky

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

@@ -3279,7 +3279,7 @@ object Types {
 
     def isGround(acc: TypeAccumulator[Boolean])(implicit ctx: Context): Boolean = {
       if (!isGroundKnown) {
-        isGroundCache = acc.foldOver(false, this)
+        isGroundCache = acc.foldOver(true, this)
         isGroundKnown = true
       }
       isGroundCache
@@ -4797,7 +4797,7 @@ object Types {
   }
 
   class isGroundAccumulator(implicit ctx: Context) extends TypeAccumulator[Boolean] {
-    def apply(x: Boolean, tp: Type) = x || {
+    def apply(x: Boolean, tp: Type) = x && {
       tp match {
         case _: TypeParamRef => false
         case tp: TypeVar => apply(x, tp.underlying)