Enable -Xsource:3

[ghostbuster91]

This should be enabled for both scala 2.12 and 2.13 builds.
It ensures that the code behaves in the same way across all language versions.

This fixes a bug in scala 2.12 and 2.13 where copy method is still accessible even though the primary constructor has been marked as private.

Above is binary incompatible change so I guess this goes into 0.19 series.

[ghostbuster91]

I will happily volunteer for this.

[kubukoz]

I would suggest checking -Xsource:3-cross, apparently it's better suited for libs that are cross-built.

[Baccata]

Go for it

[ghostbuster91]

As usual, it was easier said than done.

Turns out scala 2.12 does not support Xsource:3-cross atm. I asked if there are any plans to backport it here.

Without this we cannot use these new features like the improved semantics for copy modifiers, meaning that if we want to be better at preserving binary compatibility we have to stop using case classes in public api. Enabling it only for 2.13 & 3 doesn't make sense imo.

We could enable Xsource:3 that fails when a code with different semantics is detected but this isn't fully supported by scala 2.12 either as the following compiles without any errors/warnings:

//> using scala 2.12
//> using options -Xsource:3

case class Person private (age: Int, name: String)

[Baccata]

we have to stop using case classes in public api

I agree that ideally this should be a goal. We even had started on it, but lay-offs and reprioritisations forced us to park this effort : #1391