chore: add regression test for #24096

Author: hamzaremmal

tests/neg/i24096.check

@@ -0,0 +1,10 @@
+-- [E007] Type Mismatch Error: tests/neg/i24096.scala:8:31 -------------------------------------------------------------
+8 |        case r: Terminal[?] => r // error
+  |                               ^
+  |                     Found:    (r : Pull[F, O, Unit] & Pull.Terminal[_])
+  |                     Required: Pull[F2, O2, Unit]
+  |
+  |                     where:    F  is a type in class StreamPullOps with bounds <: [_²] =>> Any
+  |                               F2 is a type in method flatMapOutput with bounds >: [x] =>> F[x] and <: [x] =>> Any
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg/i24096.scala

@@ -0,0 +1,9 @@
+abstract class Pull[+F[_], +O, +R]
+object Pull:
+  abstract class Terminal[+R] extends Pull[Nothing, Nothing, R]
+
+  class StreamPullOps[F[_], O](self: Pull[F, O, Unit]):
+    def flatMapOutput[F2[x] >: F[x], O2](): Pull[F2, O2, Unit] =
+      self match
+        case r: Terminal[?] => r // error
+        case _              => ???