private[this] var field doesn't get updated in constructor body

[Jasper-M]

reproduction steps

using Scala 2.12.11,

scala> class Foo(private[this] var a: Int) {
     |   a = a + 1
     |   def foo = a
     | }
defined class Foo

scala> new Foo(0).foo
res0: Int = 0

problem

I mutate a in the constructor body, so I would expect the field to be mutated, not just the constructor parameter as you can see here in the bytecode (iload_1 and istore_1 instead of getfield and putfield):

scala> :javap -c -filter Foo
Compiled from "<console>"
public class Foo {
  public int foo();
    Code:
       0: aload_0
       1: getfield      #19                 // Field a:I
       4: ireturn

  public Foo(int);
    Code:
       0: aload_0
       1: iload_1
       2: putfield      #19                 // Field a:I
       5: aload_0
       6: invokespecial #26                 // Method java/lang/Object."<init>":()V
       9: iload_1
      10: iconst_1
      11: iadd
      12: istore_1
      13: return
}

This is "fixed" in Scala 2.13 but it has other issues (#12002).

The rule should be, if a field needs to be created always use the field. Otherwise always use the local parameter.

[SethTisue]

Normally we'd close the ticket since it doesn't occur in 2.13 (and there is a separate ticket for the related problem in 2.13), but in this case I'm keeping it open and tentatively milestoning it 2.12.12 so we remember to at least consider if the (eventual) fix for #12002 is backport-able and might fix this too.

[som-snytt]

#6880

[Jasper-M]

I'm sorry about the "fixed" thing 😅

[som-snytt]

One mole at a time.