private val referred to by a WeakReference should still create a field

In Scala 2, private variables in a class would be defined in the class. In Scala 3 they seem to be defined in the constructor.

I ran into this because I had weak references to the variables and then the variables got garbage collected - which was not what I wanted.

Compiler version

3.1.0-RC3

Minimized code

class Foo(i: Int) {
  private val bar: Int = 0
}

seems to give the equivalent in Java of…

public class Foo
{
    Foo(int i) {
        int bar = 0;
    }
}

when in Scala 2 it gave…

public class Foo
{
    private int bar = 0;

    Foo(int i) {
    }
}

I think Scala 2 also defined private[this] val in the class, not the constructor.

[Arthurm1]

It was something like this…

class Foo(node: javafx.scene.Node) {
  // slider is added to `node` - so still referenced - so doesn't get GC'd
  private val slider = new javafx.scene.control.ScrollBar()
  node.getChildren.add(slider)
  // `prop` is only used here and never referenced outside this code snippet
  private val prop = new javafx.beans.property.SimpleDoubleProperty()
  prop.addListener(new javafx.beans.value.ChangeListener[Number] {
    override def changed(observable: javafx.beans.value.ObservableValue[_ <: Number], oldValue: Number, newValue: Number): Unit =
      dataChanged() // local method to update UI
  })
  // bind uses weak reference to link slider and prop
  slider.valueProperty().bindBidirectional(prop)
}

JavaFX uses weak references to bind a control to a property. In this case I was binding slider to prop which then called dataChanged to update the UI. So the scrollbar would work for a few seconds and then just stopped. Took me a while to realise that prop bind was disappearing after a GC because prop was not a class field like in Scala2.

I was just going to change my code as there are various workarounds, but it would mean that I have a public getter on prop, but that isn’t a big deal.

The weak reference to an Int was me trying to minimize the example.

My suggestion of being able to write something like private[class] val prop to force prop to be a class field was so that I could ensure this doesn’t happen again in the future with prop being optimised into the constructor.

[smarter]

Ah right, WeakReference is magic, so maybe it should be special-cased by the compiler.