Upgrade default scalaVersion to 2.13

[armanbilge]

Currently the default scalaVersion in the build is Scala 2.12.

cats/build.sbt

Line 24 in 7ad08aa

ThisBuild / scalaVersion := Scala212

This is because the mdocs for the website currently do not compile on Scala 2.13. This should be investigated and fixed.

Besides other good reasons to get onto 2.13, this issue became more pressing as of the imminent Scala 2.12.16. Unfortunately it seems that Metals may have degraded support for kind-projector in Scala 2.12.16. See:

• No member completions for existential types scala/bug#12607
• chore: Add support for Scala 2.12.16 scalameta/metals#4010 (comment)

Update: Metals was testing against legacy kind-projector syntax, seems this is ok after all.

Note: we could de-couple the scalaVersion used for building docs from the default scalaVersion used for the rest of the build, but I'd rather take this opportunity to force us to fix the docs.

[tgodzik]

Unfortunately it seems that Metals may have degraded support for kind-projector in Scala 2.12.16. See:

It seems that this actually happened somewhere after 0.10.3 and also this got highlighted by Scala 2.12.16. I am not sure what is happening. I am not a macro expert but does kind projector transform the trees to something with existential types? Or maybe it's only an issue in out snippet? It's available in https://github.com/scalameta/metals/blob/main/tests/cross/src/test/scala/tests/pc/MacroCompletionSuite.scala#L167

I think that maybe existential types were never well supported in the presentation compiler and that's why this happening?