Use .superType instead of .underlying

Underlying breaks kinds (the underlying of an AppliedType is a HKTypeLambda).
The edited test case was making use of that bug to implement an
impossible match type. Indeed, that test is trying to distinguish

f[Dummy] (for some f)

and

Int

But there is no way to tell that these two types are disjoint.  In fact
they are not, since `type F[X] = Any` matches the `f[Dummy]` pattern and
overlaps with `Int`.

Author: OlivierBlanvillain

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

@@ -2508,11 +2508,11 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         !(tp2 <:< tp1)
         && (provablyDisjoint(tp1, tp2.tp2) || provablyDisjoint(tp1, tp2.tp1))
       case (tp1: TypeProxy, tp2: TypeProxy) =>
-        provablyDisjoint(tp1.underlying, tp2) || provablyDisjoint(tp1, tp2.underlying)
+        provablyDisjoint(tp1.superType, tp2) || provablyDisjoint(tp1, tp2.superType)
       case (tp1: TypeProxy, _) =>
-        provablyDisjoint(tp1.underlying, tp2)
+        provablyDisjoint(tp1.superType, tp2)
       case (_, tp2: TypeProxy) =>
-        provablyDisjoint(tp1, tp2.underlying)
+        provablyDisjoint(tp1, tp2.superType)
       case _ =>
         false
     }

tests/run-macros/tasty-simplified.check

@@ -1,4 +1,4 @@
 Functor[[A >: scala.Nothing <: scala.Any] => scala.collection.immutable.List[A]]
-Functor[Const[scala.Int]]
+Unapply[[F >: scala.Nothing <: [_$9 >: scala.Nothing <: scala.Any] => scala.Any] => Functor[F], Wrap[scala.Int]]
 Functor[Id]
 Functor[[A >: scala.Nothing <: scala.Any] => scala.Option[A]]

tests/run-macros/tasty-simplified/quoted_2.scala

@@ -7,11 +7,11 @@ object Test {
     type Id[T] = T
     case class Wrap[T](t: T)
 
-    class Dummy
+    final class Dummy
     type Apply[T[_]] = T[Dummy]
     type Unapply[F[_[_]], T] = T match {
-      case Wrap[Apply[a]] => F[a]
       case Wrap[Dummy] => F[Id]
+      case Wrap[Apply[a]] => F[a]
       case Wrap[c] => F[Const[c]]
     }