Using nn without -Yexplicit-nulls Crashes in RC1 but not M3

[AugustNagro]

Compiler version

3.0.0-RC1

(No crash in 3.0.0-M3)

Minimized code

trait MyTrait:
  def a(): String = ""

class MyClass:
  var myTrait: MyTrait|Null = null

  def printA(): Unit = println(myTrait.nn.a())

@main def runTest(): Unit =
  val mt = new MyTrait:
    override def a(): String = "hello world"

  val mc = MyClass()
  mc.myTrait = mt
  mc.printA()

Output

undefined: {
  val x$proxy1: (MyTrait | Null) = this.myTrait
  (scala.runtime.Scala3RunTime.nn[MyTrait](x$proxy1):x$proxy1.type & MyTrait)
}.a # -1: TermRef(OrType(TypeRef(ThisType(TypeRef(NoPrefix,module class <empty>)),trait MyTrait),TypeRef(TermRef(ThisType(TypeRef(NoPrefix,module class <root>)),object scala),class Null)),a) at inlining

Enabling explicit nulls fixes this, but I think the program should still compile. I ran into this problem when making an (encapsulated) Scala.js Web Component. Web Components may only have 0-arg constructors.

[AugustNagro]

Oh, I just read that Don't use .nn on mutable variables directly, because it may introduce an unknown type into the type of the variable.

Is that the case here? And would the solution be replacing nn with instanceof[MyTrait]?

[odersky]

Here's a minimization without nn or Yexplicit-nulls:

trait MyTrait:
  def a(): String = ""

class Nul

extension [T](x: T | Nul) inline def nnn: x.type & T = ???

class MyClass:
  var myTrait: MyTrait|Null = null

  def printA(): Unit = println(myTrait.nnn.a())

@main def runTest(): Unit =
  val mt = new MyTrait:
    override def a(): String = "hello world"

  val mc = MyClass()
  mc.myTrait = mt
  mc.printA()

It happens at inliner phase. Seems to be a consequence of moving inlining to its own phase.

[odersky]

In fact, things seem to work if we replace the definition of myTrait above with

var myTrait: MyTrait|Nul = ???

So it does seem to be related to Null handling in some way, after all.

Maybe a stopgap measure is to disallow nn unless -YexplicitNulls is set?

[AugustNagro]

Sounds good to me. Seems like a rare / low priority bug. I only encountered by somehow forgetting to set explicit nulls. Thanks for looking into it!

…

On Sun, Mar 7, 2021, 2:45 AM odersky ***@***.***> wrote: In fact, things seem to work if we replace the definition of myTrait above with var myTrait: MyTrait|Nul = ??? So it does seem to be related to Null handling in some way, after all. Maybe a stopgap measure is to disallow nn unless -YexplicitNulls is set? — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#11631 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ABQOKNWMFH2NIA2KN7E3KYTTCNKMLANCNFSM4YXCRMWA> .

[OlivierBlanvillain]

Closed in #13604