Simplify and fix widenAbstractTypes

Author: odersky

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

@@ -2312,18 +2312,18 @@ class TrackingTypeComparer(initctx: Context) extends TypeComparer(initctx) {
         case _ =>
           cas
       }
-      def widenAbstractTypes(tp: Type): Type = new TypeMap {
-        def apply(tp: Type) = tp match {
+      def widenAbstractTypes(tp: Type) = new TypeMap {
+        def apply(tp: Type): Type = tp match {
           case tp: TypeRef =>
-            if (tp.symbol.isAbstractOrParamType | tp.symbol.isOpaqueAlias)
-              WildcardType
-            else tp.info match {
-              case TypeAlias(alias) =>
-                val alias1 = widenAbstractTypes(alias)
-                if (alias1 ne alias) alias1 else tp
+            tp.info match {
+              case TypeBounds(lo, hi) =>
+                if (hi frozen_<:< lo) {
+                  val alias = apply(lo)
+                  if (alias ne lo) alias else mapOver(tp)
+                }
+                else WildcardType
               case _ => mapOver(tp)
             }
-
           case tp: TypeVar if !tp.isInstantiated => WildcardType
           case _: TypeParamRef => WildcardType
           case _ => mapOver(tp)