Dotty cross-building [WIP]

[travisbrown]

This is a work-in-progress pull request that should pass CI but is missing or waiting on several things:

• Official Dotty release of ScalaTest.
• Official Dotty release of the ScalaTest ScalaCheck compatibility module.
• An official decision on what to do about Simulacrum.
• An official decision on what to do about use of kind-projector's polymorphic lambda value syntax.
• Also I just haven't bothered to migrate cats-free (which uses existential types) yet.

Note that while this PR looks like a big change—currently +2,662 -297—over 2,400 of the added lines have been automatically generated by Scalafix rules, and another hundred or so are just moving the FunctionK.lift macro into version-specific code. There aren't much more than a hundred lines of substantive changes.

I've been opening Dotty bug reports for issues that I've had to work around. Some of these have already been fixed, and it's likely that more will be resolved by the time this PR is ready to merge:

• "Ambiguous" imports collide with object definitions scala/scala3#7876
• Illegal access error when constructor parameter has same name and more specific type scala/scala3#7926
• Type alias causes problems for implicit search scala/scala3#7993
• Spurious implicit argument ambiguity scala/scala3#7999
• Bridge method clash with value class scala/scala3#8001
• Serializing val that refers to another val captures enclosing class scala/scala3#8033 (but still needs workaround since this was merged since 0.22.0-RC1)
• ClassCastException on by-name parameter in enrichment method scala/scala3#8035
• Implicit conversion isn't applied for Id type alias scala/scala3#8049
• Possibly spurious type test warning scala/scala3#8383 (not necessary but it means we have to disable fatal warnings on Dotty)

I'm opening this for tracking and discussion now that -Ykind-projector is in the Dotty nightly releases, but we probably won't be in a position to merge it for a while, and I'll be rebasing and force-pushing to keep it updated.

I have a similar branch for cats-effect, but haven't opened a PR there yet (update: WIP cats-effect PR is open).

[codecov-io]

Codecov Report

Merging #3269 into master will decrease coverage by 1.15%.
The diff coverage is 64.87%.

@@            Coverage Diff             @@
##           master    #3269      +/-   ##
==========================================
- Coverage   93.31%   92.15%   -1.16%     
==========================================
  Files         378      378              
  Lines        7684     7932     +248     
  Branches      233      206      -27     
==========================================
+ Hits         7170     7310     +140     
- Misses        514      622     +108

Flag	Coverage Δ
#scala_version_212	92.18% <64.1%> (-1.21%)	⬇️
#scala_version_213	91.94% <64.87%> (-1.15%)	⬇️

Impacted Files	Coverage Δ
...rc/main/scala/cats/laws/NonEmptyTraverseLaws.scala	85% <ø> (ø)	⬆️
core/src/main/scala/cats/CommutativeMonad.scala	0% <0%> (ø)	⬆️
core/src/main/scala/cats/CommutativeApply.scala	50% <0%> (-16.67%)	⬇️
core/src/main/scala/cats/Monad.scala	81.25% <0%> (-15.05%)	⬇️
core/src/main/scala/cats/Distributive.scala	50% <0%> (-16.67%)	⬇️
core/src/main/scala/cats/CommutativeFlatMap.scala	0% <0%> (ø)	⬆️
...e/src/main/scala/cats/CommutativeApplicative.scala	60% <0%> (-15%)	⬇️
...lleycats-core/src/main/scala/alleycats/ConsK.scala	0% <0%> (ø)	⬆️
...eycats-core/src/main/scala/alleycats/Extract.scala	0% <0%> (ø)	⬆️
alleycats-core/src/main/scala/alleycats/Pure.scala	0% <0%> (ø)	⬆️
... and 70 more

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[larsrh]

Obsoleted by #3486