Fix unreachable warning in deeply nested sealed hierarchy

Without applying we were constructing the type Jacket#Body, instead of
Jacket[?]#Body, which lead down to an incorrect isSubSpace calculation
and thus an unreachable warning.

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

@@ -883,7 +883,7 @@ object TypeOps:
           else if symbol.is(Module) then
             TermRef(this(tref.prefix), symbol.sourceModule)
           else if (prefixTVar != null)
-            this(tref)
+            this(tref.applyIfParameterized(tref.typeParams.map(_ => WildcardType)))
           else {
             prefixTVar = WildcardType  // prevent recursive call from assigning it
             // e.g. tests/pos/i15029.more.scala, create a TypeVar for `Instances`' B, so we can disregard `Ints`

tests/warn/i18661.scala

@@ -0,0 +1,28 @@
+class Jacket[T]:
+  sealed trait BodyType:
+    sealed trait OrganType:
+      case class Heart() extends Body.Organ
+      case class Brain() extends Body.Organ
+    object Organ extends OrganType
+    sealed trait Organ
+  object Body extends BodyType
+  sealed trait Body
+
+type AnyJacket         = Jacket[?]
+type AnyBodyOrgan      = AnyJacket#BodyType#Organ
+type AnyBodyOrganHeart = AnyJacket#BodyType#OrganType#Heart
+type AnyBodyOrganBrain = AnyJacket#BodyType#OrganType#Brain
+
+def check( asr : AnyBodyOrgan ) : String =
+  asr match
+    case c : AnyBodyOrganHeart => "Heart"
+    case s : AnyBodyOrganBrain => "Brain" // was: unreachable
+
+val jacket = new Jacket[Unit]
+val heart = new jacket.Body.Organ.Heart()
+val brain = new jacket.Body.Organ.Brain()
+
+@main
+def go =
+  println( check( heart ) )
+  println( check( brain ) )