Welcome to Scala 3.6.3 (21.0.5, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.
                                                                                                                                           
scala> val a: List[_] = Nil
1 warning found
-- Warning: --------------------------------------------------------------------
1 |val a: List[_] = Nil
  |            ^
  |        `_` is deprecated for wildcard arguments of types: use `?` instead
val a: List[?] = List()
                                                                                                                                           
scala> val b: List[?] = Nil
val b: List[?] = List()