Add regression test for scala#14821

tests/pos/i14821.scala

@@ -0,0 +1,29 @@
+trait Statement
+trait Definition extends Statement
+
+trait ClassDef extends Definition:
+  def constructor: DefDef
+
+object ClassDef:
+  def copy(constr: DefDef): ClassDef = ???
+
+// >>> This abstract implementation of DefDef causes a compilation error in transform...
+type DefDef <: Definition
+val DefDef: DefDefModule = ???
+trait DefDefModule:
+  def unapply(ddef: DefDef): (String, List[AnyRef])
+// ...unless this given TypeTest is commented out, in which case we get only a type test warning
+given scala.reflect.TypeTest[Statement, DefDef] = ???
+
+// >>> This alternative works
+// trait DefDef extends Definition
+// object DefDef:
+//   def unapply(ddef: DefDef): (String, List[AnyRef]) = ???
+
+// >>> This alternative also works
+// case class DefDef(name: String, paramss: List[AnyRef]) extends Definition
+
+def transform(tree: Statement): Statement = tree match
+  case tree: ClassDef =>
+    val constructor @ DefDef(_, _) = transform(tree.constructor): @unchecked
+    ClassDef.copy(constructor)