Add a pos test for #9504

Thanks to the previous commit, this succeeds instead of looping in
implicit search.

Author: smarter

tests/pos/i9504.scala

@@ -0,0 +1,15 @@
+trait Monad[F[_]] {
+  def foo[A](fa: F[A]): Unit = {}
+}
+
+class Bla[F[_], A]
+
+object Test {
+  type Id[A] = A
+
+  val bla: Bla[Id, Unit] = ???
+  implicit def blaMonad[F[_]: Monad, S]: Monad[({type L[X] = Bla[F, X]})#L] = ???
+  implicit def idMonad: Monad[Id] = ???
+
+  blaMonad.foo(bla) // does not diverge
+}