Compiler options -Ysafe-init and -Yexplicit-nulls do not report unsafe null inits in classes

Compiler version

3.0.2

Minimized code

class NullFoo:
  var s: String = _
  override def toString: String = s"Foo:s=$s"

object NullFoo:
  def main(args: Array[String]): Unit =
    println(new NullFoo)

Output

Foo:s=null

Expectation

With these compiler options, I would expect a compile error:

scalacOptions ++= Seq(
  "-Ysafe-init", // renamed from -Ycheck-init, still present in official docs!
  "-Yexplicit-nulls",
  "-Xfatal-warnings"
)

See also https://stackoverflow.com/questions/69612468/scala-3-ysafe-init-and-yexplicit-nulls-do-not-report-unsafe-null-inits-in-clas

[olhotak]

This was discussed in the LAMP meeting today. The consensus there was that:

var x: String = _ should trigger a deprecation warning.

var x: String = uninitialized should not trigger a warning because otherwise all real-world uses of uninitialized would just be annotated with @unchecked, which is an unnecessary annoyance.

[ebruchez]

Today I set to demo the great Scala 3 null-safety abilities to someone, so I set:

"-Yexplicit-nulls",
"-Xfatal-warnings"

I wrote the following code in Scastie:

class D:
  var x: String = _

println(D().x)

And of course I got a null printed out, which is what this issue is about. It’s a disappointing first encounter with null-safety, I have to say! So +1 to do something about it really soon.

This said, why would:

var x: String = _

only trigger a deprecation warning? After all, I enabled explicit nulls AND safe initialization. I’d prefer it if this was an error fair and square when one or both of these option are enabled.