Reduce match types in superType instead of translucentSuperType

Author: odersky

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

@@ -1881,7 +1881,9 @@ object Types {
     /** The closest supertype of this type. This is the same as `underlying`,
      *  except that
      *    - instead of a TyperBounds type it returns its upper bound, and
-     *    - for applied types it returns the upper bound of the constructor re-applied to the arguments.
+     *    - for applied types it returns
+     *       * if the applied type can be matchtype-reduced, the redux
+     *       * otherwise, the upper bound of the constructor re-applied to the arguments.
      */
     def superType(using Context): Type = underlying match {
       case TypeBounds(_, hi) => hi
@@ -3996,12 +3998,15 @@ object Types {
 
     override def superType(using Context): Type = {
       if (ctx.period != validSuper) {
-        cachedSuper = tycon match {
-          case tycon: HKTypeLambda => defn.AnyType
-          case tycon: TypeRef if tycon.symbol.isClass => tycon
-          case tycon: TypeProxy => tycon.superType.applyIfParameterized(args)
-          case _ => defn.AnyType
-        }
+        cachedSuper = tycon match
+          case tycon: HKTypeLambda =>
+            defn.AnyType
+          case tycon: TypeRef if tycon.symbol.isClass =>
+            tycon
+          case tycon: TypeProxy =>
+            tryNormalize.orElse(tycon.superType.applyIfParameterized(args))
+          case _ =>
+            defn.AnyType
         validSuper = if (tycon.isProvisional) Nowhere else ctx.period
       }
       cachedSuper
@@ -4011,7 +4016,7 @@ object Types {
       case tycon: TypeRef if tycon.symbol.isOpaqueAlias =>
         tycon.translucentSuperType.applyIfParameterized(args)
       case _ =>
-        tryNormalize.orElse(superType)
+        superType
     }
 
     inline def map(inline op: Type => Type)(using Context) =