Weird case of double-parentheses impacting type inference in List.apply

[neko-kai]

Minimized code

class X

object X {
  extension (x: List[X]) { def isNull = x.head == null }
}

@main def test = List(null: X).isNull

https://scastie.scala-lang.org/He9Pm96ySZCBCziHLktlXg

Output

Error:

Found:    X
Required: Any*

Expectation

Should compile. If a redundant second set of parentheses is added in List((null: X)), then the example compiles:

class X

object X {
  extension (x: List[X]) { def isNull = x.head == null }
}

@main def test = List((null: X)).isNull

https://scastie.scala-lang.org/dwFH7NAsQyqKpoXHuzAWiA

[som-snytt]

scala> List(null: String)
1 |List(null: String)
  |     ^^^^^^^^^^^^
  |     Found:    String
  |     Required: Any*

scala> List("": String)
val res4: List[String] = List("")

scala> List((null: Seq[String]): _*)
java.lang.NullPointerException: Cannot invoke "scala.collection.IterableOnce.knownSize()" because "prefix" is null

Scala 2 doesn't fare as well with List(null: _*).

[neko-kai]

@som-snytt
Hmm, works for me on Scala 2.13.3, your first repl command & scastie (https://scastie.scala-lang.org/yYKd7yxWTdSeKF1wezvFYQ).

class X

object App extends App {
  val test = List(null: X)
  println(test)
}

Is your repl printout from 2.12?

[som-snytt]

Sorry I didn't add commentary. It's null sequence arg that is the problem for scala 2, which scala 3 handles the same as if it were null: Seq[_].

I was curious why it says Any* is required. The sequence arg annotation is just an ascription that can only be applied to the last argument in an application. I was curious about other positional problems and opened the linked ticket.

The other point was that the error here happens on List(null: String), which scala 2 is fine with. Maybe scala 3 already checked null <:< Seq[_]? I would think you have to look at _* even to understand what the application is, see the linked ticket.