Widen skolem in scrutinee when reducing or comparing match types

Co-Authored-By: Dale Wijnand <dale.wijnand@gmail.com>

Author: mbovel

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

@@ -1007,7 +1007,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
       case tp1: MatchType =>
         def compareMatch = tp2 match {
           case tp2: MatchType =>
-            isSameType(tp1.scrutinee, tp2.scrutinee) &&
+            isSameType(tp1.scrutinee.widenSkolem, tp2.scrutinee.widenSkolem) &&
             tp1.cases.corresponds(tp2.cases)(isSubType)
           case _ => false
         }

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

@@ -4954,7 +4954,7 @@ object Types {
           trace(i"reduce match type $this $hashCode", matchTypes, show = true) {
             def matchCases(cmp: TrackingTypeComparer): Type =
               val saved = ctx.typerState.snapshot()
-              try cmp.matchCases(scrutinee.normalized, cases)
+              try cmp.matchCases(scrutinee.normalized.widenSkolem, cases)
               catch case ex: Throwable =>
                 handleRecursive("reduce type ", i"$scrutinee match ...", ex)
               finally