Disallow Array[Nothing], Array[Null], ... to avoid ClassCastException in some cases

[smarter]

See also https://issues.scala-lang.org/browse/SI-7453

In scalac:

scala> if (false) Array("qwe") else Array()
java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.String;

In dotty, thankfully:

scala> if (false) Array("qwe") else Array() 
res1: Array[String] | Array[Nothing] = [Ljava.lang.Object;@1dd92fe2

But still in dotty:

scala> val x = if (false) Array("qwe") else Array() 
x: Array[String] | Array[Nothing] = [Ljava.lang.Object;@5454d35e
scala> val y: Array[_ <: String] = x 
java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.String;

We either need to disallow Array[Nothing], Array[Null], etc or we need to erase Array[_ <: X] to Object instead of [X;.

It would also be much nicer if we could infer the empty array above to have type Array[String] instead of Array[Nothing], I don't know what prevents us from doing this currently.

[odersky]

Erasing Array[_ <: X] to Object is very expensive computationally, and it would be surprising for users. So I am much in favor of disallowing Array[Nothing] and Array[Null].

[odersky]

One quite radical idea would be to treat Nothing and Null not as classes but as abstract types. That means we could not generate a ClassTag for them and therefore it would be impossible to create instances of Array[Nothing] and Array[Null]`

[sjrd]

If we disallow classTag[Null], then we should also disallow classOf[Null]. This is extreme, as now we have concrete types we cannot a classOf/classTag of anymore. "Treating them as abstract types" seems completely unjustified, other than as a workaround for this implementation limitation related to arrays. This seems wrong to me.

[odersky]

Is classOf[Null] used anywhere?

I have my doubts because in the end there is no class Null on the JVM. We fake one, but it's a hack.

[retronym]

Here are a few usages I found in projects I've checked out. Seems fairly rare.

⚡ ack --scala 'classOf\[Null\]'

json4s/scalabeans/src/main/scala/org/scalastuff/scalabeans/sig/ScalaTypeCompiler.scala
117:    else if (clazz == "scala.Null") Some(classOf[Null])

scalatra/scalatra/swagger/src/main/scala/org/scalatra/swagger/reflect/Reflector.scala
26:  private[this] val defaultExcluded = Set(classOf[Nothing], classOf[Null])

twitter/util/util-reflect/src/main/scala/com/twitter/util/reflect/Proxy.scala
89:    (rt eq classOf[Unit]) || (rt eq classOf[Null]) || (rt eq java.lang.Void.TYPE)

[nicolasstucki]

Currently in Dotty we have

scala> if (false) Array("qwe") else Array() 
-- Error: <console>:5:36 -------------------------------------------------------
5 |if (false) Array("qwe") else Array()
  |                                    ^
  |attempt to take ClassTag of undetermined type for parameter evidence$2 of method apply in object Array

@odersky can this issue be closed? This was probably fixed in e34555f

[smarter]

@nicolasstucki No, as stated in the PR that this commit comes from: #1921 (comment)

[odersky]

The comment says that we might want to consider disallowing forming a ClassTag[Null] or ClassTag[Nothing] manually (they are not synthesized anymore). On the other hand, anyone who would do that in a sense does homegrown meta-programming, and it's not clear to what extent we want to give soundness guarantees for such code. So I think it's OK to close for now. If we feel like it, revisit the manual creation issue.