Eta-expansion, via trailing underscore, of methods with no argument lists is now deprecated

[dwijnand]

Eta-expansion of nullary methods is dropped in Scala 3:
https://dotty.epfl.ch/docs/reference/changed-features/eta-expansion.html

So it must be deprecated in 2.13 first, and error under -Xsource:3.

[lrytz]

Implementation LGTM, and I think this we can definitely ship in a 2.13 minor release.

[lrytz]

LGTM. For the record, the restriction was added in scala/scala3@10ee00e.

@SethTisue wdyt about merging this in 2.13.2?

[dwijnand]

Note this will have an associated Scalafix rewrite.

[SethTisue]

wdyt about merging this in 2.13.2?

Let's do it.

(I waffled some before answering — we've already asked people to test nightlies, so anything we merge at this point missed that boat, which makes me a bit nervous, even though this particular change doesn't seem risky. But then I thought: 2.13.2 is already a pretty big release for a minor release, and 2.13.1 was solid, so people can safely stay there a bit longer if they need to. So on the balance, we are on a somewhat risky road already, so I think we might as well pile on, and if it means we end up having to fast-track a 2.13.3 release, then so be it. We have to take the risks sometime, the changes have to go in some release.)

[lrytz]

Note this will have an associated Scalafix rewrite.

That would be nice! It would give us more experience about suggesting Scalafix for changes in minor releases, which will be more crucial when we do #8833 / #8846.

[dwijnand]

Scalafix rewrite: lightbend-labs/scala-rewrites#15