fix #9260 - Enums copy variance of parent

tests/neg-custom-args/fatal-warnings/enum-variance.check

@@ -0,0 +1,5 @@
+-- Error: tests/neg-custom-args/fatal-warnings/enum-variance.scala:2:12 ------------------------------------------------
+2 |  case Refl(f: T => T) // error: covariant T in contravariant position... enum case may require explicit type parameters
+  |            ^^^^^^^^^
+  |            covariant type T occurs in contravariant position in type T => T of value f
+  |            enum case class Refl may require explicit type parameters to resolve this issue

tests/neg-custom-args/fatal-warnings/enum-variance.scala

@@ -0,0 +1,2 @@
+enum View[+T]:
+  case Refl(f: T => T) // error: covariant T in contravariant position... enum case may require explicit type parameters

tests/pos-special/fatal-warnings/i9260.scala

@@ -0,0 +1,16 @@
+package asts
+
+enum Ast[-T >: Null]:
+  case DefDef()
+
+trait AstImpl[T >: Null]:
+  type Ast = asts.Ast[T]
+  type DefDef = Ast.DefDef[T]
+end AstImpl
+
+object untpd extends AstImpl[Null]:
+
+  def DefDef(ast: Ast): DefDef = ast match
+    case ast: DefDef => ast
+
+end untpd

tests/pos/enum-variance.scala

@@ -0,0 +1,18 @@
+class Animal
+class Dog extends Animal
+
+enum Opt[+T]:
+  case Sm(t: T)
+  case None
+
+val smDog: Opt.Sm[Dog] = new Opt.Sm(Dog())
+val smAnimal: Opt.Sm[Animal] = smDog
+
+enum Show[-T]:
+  case Refl(op: T => String)
+
+  def show(t: T): String = this match
+    case Refl(op) => op(t)
+
+val reflAnimal: Show.Refl[Animal] = new Show.Refl(_.toString)
+val reflDog: Show.Refl[Dog] = reflAnimal