Make normalizing superTypes valid nowhere

Author: OlivierBlanvillain

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

@@ -4105,18 +4105,20 @@ object Types {
 
     override def underlying(using Context): Type = tycon
 
-    override def superType(using Context): Type = {
-      if (ctx.period != validSuper) {
-        cachedSuper = tycon match {
+    override def superType(using Context): Type =
+      if ctx.period != validSuper then
+        validSuper = if (tycon.isProvisional) Nowhere else ctx.period
+        def reappliedSuper = tycon.superType.applyIfParameterized(args).normalized
+        cachedSuper = tycon match
           case tycon: HKTypeLambda => defn.AnyType
-          case tycon: TypeRef if tycon.symbol.isClass => tycon
-          case tycon: TypeProxy => tycon.superType.applyIfParameterized(args).normalized
+          case tycon: TypeRef =>
+            if tycon.symbol.isClass then tycon
+            else
+              if tycon.info.isInstanceof[MatchAlias] then validSuper = Nowhere
+              reappliedSuper
+          case tycon: TypeProxy => reappliedSuper
           case _ => defn.AnyType
-        }
-        validSuper = if (tycon.isProvisional) Nowhere else ctx.period
-      }
       cachedSuper
-    }
 
     override def translucentSuperType(using Context): Type = tycon match {
       case tycon: TypeRef if tycon.symbol.isOpaqueAlias =>