Fix derivedRefinedType in ApproximatingTypeMap

The logic before was overcomplicated since I did not take into
account that Range refinedInfos could only happen at variance 0.
On the othet hand, it did not take into account all the subtleties of
alias types with variances.

Author: odersky

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

@@ -3923,20 +3923,20 @@ object Types {
         case Range(parentLo, parentHi) =>
           range(derivedRefinedType(tp, parentLo, info), derivedRefinedType(tp, parentHi, info))
         case _ =>
+          def propagate(lo: Type, hi: Type) =
+            range(derivedRefinedType(tp, parent, lo), derivedRefinedType(tp, parent, hi))
           if (parent.isBottomType) parent
           else info match {
+            case Range(infoLo: TypeBounds, infoHi: TypeBounds) =>
+              assert(variance == 0)
+              val v1 = infoLo.variance
+              val v2 = infoHi.variance
+              if (v1 > 0 && v2 > 0) propagate(infoLo, infoHi)
+              else if (v1 < 0 && v2 < 0) propagate(infoHi, infoLo)
+              else if (!infoLo.isAlias && !infoHi.isAlias) propagate(infoLo, infoHi)
+              else range(tp.bottomType, tp.topType)
             case Range(infoLo, infoHi) =>
-              def propagate(lo: Type, hi: Type) =
-                range(derivedRefinedType(tp, parent, lo), derivedRefinedType(tp, parent, hi))
-              tp.refinedInfo match {
-                case rinfo: TypeBounds =>
-                  val v = if (rinfo.isAlias) rinfo.variance * variance else variance
-                  if (v > 0) tp.derivedRefinedType(parent, tp.refinedName, rangeToBounds(info))
-                  else if (v < 0) propagate(infoHi, infoLo)
-                  else range(tp.bottomType, tp.topType)
-                case _ =>
-                  propagate(infoLo, infoHi)
-              }
+              propagate(infoLo, infoHi)
             case _ =>
               tp.derivedRefinedType(parent, tp.refinedName, info)
           }