Fix scala#9533: handle nested TypeParamRef in bounds

Author: liufengyun

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

@@ -224,10 +224,13 @@ final class ProperGadtConstraint private(
   override protected def isSame(tp1: Type, tp2: Type)(using Context): Boolean = TypeComparer.isSameType(tp1, tp2)
 
    override def nonParamBounds(param: TypeParamRef)(using Context): TypeBounds =
-     constraint.nonParamBounds(param) match {
-       case TypeAlias(tpr: TypeParamRef) => TypeAlias(externalize(tpr))
-       case tb => tb
+     val externalizeMap = new TypeMap {
+       def apply(tp: Type): Type = tp match {
+         case tpr: TypeParamRef => externalize(tpr)
+         case tp => mapOver(tp)
+       }
      }
+     externalizeMap(constraint.nonParamBounds(param)).bounds
 
    override def fullLowerBound(param: TypeParamRef)(using Context): Type =
      constraint.minLower(param).foldLeft(nonParamBounds(param).lo) {

tests/pos/i9533.scala

@@ -0,0 +1,10 @@
+object Test {
+  sealed trait Base[-X]
+  case class Child[X]() extends Base[X]
+
+  def apply[A, B](r: Base[A with B]): Unit = {
+    r match {
+      case Child() => ()
+    }
+  }
+}