Regression test for #8984 (#16100)

Fixes #8984, in the sense that it crashed before and now an error is
reported.

Author: odersky

tests/neg/i8984.check

@@ -0,0 +1,9 @@
+-- [E007] Type Mismatch Error: tests/neg/i8984.scala:36:52 -------------------------------------------------------------
+36 |            case Cat(_, "red", rest) => dropRedCats(rest)  // error
+   |                                                    ^^^^
+   |                               Found:    (rest : Any)
+   |                               Required: Fix.T[F]
+   |
+   |                               where:    F is a type in method dropRedCats with bounds >: Cat and <: [a] =>> Any
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg/i8984.scala

@@ -0,0 +1,61 @@
+import scala.annotation.tailrec
+type |@[F[+_], G[+_]] = [a] =>> F[a] | G[a]
+
+object Fix:
+  opaque type T[+F[+_]] = ApplyFix.T[F]
+
+  def apply[F[+_]](f: F[Fix[F]]): T[F] = ApplyFix(f)
+
+  extension [F[+_]](fix: T[F])
+    def value: F[Fix[F]] = ApplyFix.unwrap(fix)
+
+  object ApplyFix:
+    opaque type T[+F[+_]] = F[Fix[F]]
+
+    def apply[F[+_]](f: F[Fix[F]]): T[F] = f
+
+    def unwrap[F[+_]](v: T[F]): F[Fix[F]] = v
+
+type Fix[+F[+_]] = Fix.T[F]
+
+final case class Cat[+R](name: String, fur: String, rest: R)
+object Cat:
+    def of[R, F[+_]](name: String, fur: String, rest: Fix[F]): Fix[F |@ Cat] = Fix(new Cat(name, fur, rest))
+
+final case class Dog[+R](name: String, size: Long, rest: R)
+object Dog:
+    def of[R, F[+_]](name: String, size: Long, rest: Fix[F]): Fix[F |@ Dog] = Fix(new Dog(name, size, rest))
+
+case object End:
+    type f[+a] = End.type
+    def apply() = Fix[f](End)
+
+object DropRed:
+    @tailrec def dropRedCats[F[+a] >: Cat[a]](cats: Fix[F]): Fix[F] =
+        cats.value match
+            case Cat(_, "red", rest) => dropRedCats(rest)  // error
+            case _ => cats
+
+    type CatDogVector = Vector[Either[Cat[Unit], Dog[Unit]]]
+    type CatOrDogs[+a] =  Cat[a] | Dog[a] | End.type
+
+    extension (catDogs: Fix[CatOrDogs]) def toVector : CatDogVector  =
+        @tailrec def go(acc: CatDogVector, catDogs: Fix[CatOrDogs]) : CatDogVector = catDogs.value match
+            case Cat(name, fur, rest) =>  go(acc :+ Left(Cat(name, fur, ())), rest)
+            case Dog(name, size, rest) => go(acc :+ Right(Dog(name, size, ())), rest)
+            case End => acc
+
+        go(Vector(), catDogs)
+
+    val x =
+        Cat.of("lilly" , "red"  ,
+        Cat.of("anya"  , "red"  ,
+        Cat.of("boris" , "black",
+        Dog.of("mashka", 3      ,
+        Cat.of("manya" , "red"  ,
+        End())))))
+
+
+    def main(args: Array[String]) =
+        println(x.toVector)
+        println(dropRedCats(x).toVector)