Disallow type Null in a type pattern or isInstanceOf test

Should we disallow type Null in a type pattern or isInstanceOf test like scalac does to avoid surprising behaviors?

Dotty:

scala> null.isInstanceOf[Null]
val res0: Boolean = false

Scalac:

scala> null.isInstanceOf[Null]
<console>:12: error: type Null cannot be used in a type pattern or isInstanceOf test
       null.isInstanceOf[Null]
                        ^

[smarter]

I would say yes, same with Singleton.

[Blaisorblade]

The one with Null might be useful in cases where an isInstanceOf check is inserted by the compiler for some reason (not sure if it ever happens).

So somebody could just forbid isInstanceOf[Null] and see if something breaks. Volunteers? @liufengyun maybe in your PR when you have time?

Pattern matching with singleton types (which is equivalent to isInstanceOf, IIUC) is useful (at least a bit) with GADTs:

def f[T, U](t: Foo[T], u: Foo[U]) =
  t match {
    case _: u.type => // Here T and U are equal
  }

An actual use case is in ECOOP’07 Burak Emir’s paper on case classes, and that code still works today in Scala (though IIRC not yet in Dotty).