Case class copy and apply inherit access modifiers from constructor (under -Xsource:3)

[Jasper-M]

Backport from dotty:

• If a case class constructor is private or private[foo]: the synthesized copy and apply methods will have the same access modifier.
• If a case class constructor is protected or protected[foo]: the synthesized copy method will have the same access modifier. The synthesized apply method will remain public, because protected doesn't make sense in an object.

Obviously, if a user defines a custom copy or apply method, that one—including its access modifier—will take precedence.

See the accompanying pos and neg tests.

Only enabled under -Xsource:2.14 and by extension under -Xsource:3.

[Jasper-M]

Added some changes to bring the PR in line with scala/scala3#5830 and fixes for corresponding dotty issues scala/scala3#5857 and scala/scala3#5859.

[Jasper-M]

@adriaanm All those private constructors in pos/userdefined_apply.scala, are they there to avoid that calls to apply get transformed to calls to the constructor? In which case it would be fine to make them protected instead?

[adriaanm]

@adriaanm All those private constructors in pos/userdefined_apply.scala, are they there to avoid that calls to apply get transformed to calls to the constructor? In which case it would be fine to make them protected instead?

IIRC it's to control who can create an instance of a class -- if this is what you want, a private constructor is useless if your apply is public, so basically the same as what your PR offers by default :-)

[Jasper-M]

@adriaanm Err... Not following 100% what you're saying. Some of those tests seem to rely specifically on the generated apply not being private.

But anyway I think this PR is ready for review and discussion on how to proceed (SIP or no SIP, -Xsource flag or no flag, ...).

Status of this PR right now:

• private and protected, both qualified and unqualified, get inherited by the copy method from the constructor
• (un)qualified private gets inherited by the apply method from the constructor
• (un)qualified protected constructor results in normal public apply method, as seen in dotty
• case classes with a (un)qualified private constructor will have a companion object that does not extend a FunctionN trait.

I reviewed all failing tests and the bugs they are defending against. In all cases the right thing to do seemed to me turning private constructors into protected. That way apply is still public and calls to apply will not get rewritten into calls to the constructor.

[eed3si9n]

But anyway I think this PR is ready for review and discussion on how to proceed (SIP or no SIP, -Xsource flag or no flag, ...).

@smarter Is there a SIP for this on Dotty's change?

[smarter]

@smarter Is there a SIP for this on Dotty's change?

There isn't.

[tpolecat]

I wasn't aware of this PR until just now. It would be really nice to get this into Scala 2 so we can get away from sealed abstract case class (although I admit it's probably my favorite trick).

[Jasper-M]

Added some tests based on @eed3si9n's comment. And rebased in the hopes of fixing MIMA failures in travis.

[Jasper-M]

Ok, I get it now. MIMA complains because some apply and copy methods became private.

[Jasper-M]

I put the change behind the -Xsource:2.14 flag, since this can't go in 2.13 anyway. That should also fix the MIMA issues.

[lrytz]

This looks really good! Implementation, testing, all 👍.

I think this doesn't need a SIP.

My only concern is that it's a breaking change, but I cannot think of anything that we can do in 2.13 to aid / anticipate migration.

[Jasper-M]

Can this be considered for 2.13.2?

[lrytz]

Under -Xsource:2.14 for sure. I didn't merge it so @adriaanm can take another look.

[adriaanm]

Sorry for the delay. I will do my best to look soon. In principle it all looks good.

[adriaanm]

Sorry for lagging on this. I've asked Jason to take a look instead. He's been reworking this part of namers.

[SethTisue]

@retronym do you intend to try to get this over the finish line for 2.13.2?

[lrytz]

Let's merge this, it's looking good