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

@@ -3927,20 +3927,27 @@ 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
+              // There's some weirdness coming from the way aliases can have variance
+              // If infoLo and infoHi are both aliases with the same non-zero variance
+              // we can propagate to a range of the refined types. If they are both
+              // non-alias ranges we know that infoLo <:< infoHi and therefore we can
+              // propagate to refined types with infoLo and infoHi as bounds.
+              // In all other cases, Nothing..Any is the only interval that contains
+              // the range. i966.scala is a test case.
+              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)
           }