Assess `-Xsource:3` using community build

[SethTisue]

This came up with @smarter at https://gitter.im/typelevel/general?at=6086d87e81866c680c48df4f

I've recently added a bunch of Scala 3 syntax under that flag to allow us to deprecate and potentially repurpose some old syntax without harming cross-compatibility with Scala 2, but this hinges on people actually using that flag so it'd be good to know if there's any issues with it

We should at least try it and see what happens.

[SethTisue]

@smarter the behemoths are busy today with the sbt 1.5.1 upgrade, but I should be able to do this in the next few days

[SethTisue]

https://scala-ci.typesafe.com/job/scala-2.13.x-jdk11-integrate-community-build/2703/

42 green repos, but also lots of failures, most notably scalatest,kind-projector,utest,shapeless,scalatest-3-0; transitively, those prevent most of the build from running

sampling of failure causes:

• scalatest: "method without a parameter list overrides a method with a single empty one"
• kind-projector: "Either[Int, _] takes no type parameters, expected: 1", I suppose because it's using the old wildcard syntax
• utest: "symbol literal is unsupported; use Symbol("hello") instead"
• shapeless: "Malformed literal or standard type 'a"

fixes for these could easily be submitted upstream, and then we could re-run and get more results

this is one of those times where we don't primarily want a community build that rewires all the dependencies; we would primarily want to build each repo separately

it would not be super hard for me (or somebody) to write a script that glues all the proj/*.conf files together in an alternate why where each repo is built in its own space, with check-missing: false so that all dependencies would be retrieved from Maven Central et al. at least, I don't think I would run into a bunch of weird trouble doing that

on the other hand, I don't have any further time for this either this week or next, but I could probably return to it after that

[smarter]

fixes for these could easily be submitted upstream

I'll see if I can look into it. I think the compiler could also stop emitting errors on symbol sytnax since it's still supported in Scala 3 under a language feature. But for kind-projector at least, the error likely comes from a test exercising the ? syntax since it's still supported even if it's deprecated, and upstream might not want to get rid of it right now.

on the other hand, I don't have any further time for this either this week or next, but I could probably return to it after that

That would be great! No pressure though :).

[smarter]

• Don't error (only warn) on symbol literals under -Xsource:3 scala#9602
• Fix compilation with -Xsource:3 scalatest/scalatest#2020
• Remove deprecated ? syntax typelevel/kind-projector#186

[smarter]

@SethTisue do you think you could try running this again now that all the known issues are fixed?

[SethTisue]

@smarter sure, after scala/community-build#1418 lands