Space: Make isDecomposableToChildren ignore type constructors

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -642,7 +642,7 @@ object SpaceEngine {
         // For instance, from i15029, `decompose((X | Y).Field[T]) = [X.Field[T], Y.Field[T]]`.
         parts.map(tp.derivedAppliedType(_, targs))
 
-      case tp if tp.classSymbol.isDecomposableToChildren =>
+      case tp if tp.isDecomposableToChildren =>
         def getChildren(sym: Symbol): List[Symbol] =
           sym.children.flatMap { child =>
             if child eq sym then List(sym) // i3145: sealed trait Baz, val x = new Baz {}, Baz.children returns Baz...
@@ -678,8 +678,8 @@ object SpaceEngine {
     rec(tp, Nil)
   }
 
-  extension (cls: Symbol)
-    /** A type is decomposable to children if it's sealed,
+  extension (tp: Type)
+    /** A type is decomposable to children if it has a simple kind, it's sealed,
       * abstract (or a trait) - so its not a sealed concrete class that can be instantiated on its own,
       * has no anonymous children, which we wouldn't be able to name as counter-examples,
       * but does have children.
@@ -688,7 +688,8 @@ object SpaceEngine {
       * A sealed trait with subclasses that then get removed after `refineUsingParent`, decomposes to the empty list.
       * So that's why we consider whether a type has children. */
     def isDecomposableToChildren(using Context): Boolean =
-      cls.is(Sealed) && cls.isOneOf(AbstractOrTrait) && !cls.hasAnonymousChild && cls.children.nonEmpty
+      val cls = tp.classSymbol
+      tp.hasSimpleKind && cls.is(Sealed) && cls.isOneOf(AbstractOrTrait) && !cls.hasAnonymousChild && cls.children.nonEmpty
 
   val ListOfNoType    = List(NoType)
   val ListOfTypNoType = ListOfNoType.map(Typ(_, decomposed = true))