Avoid infix, tuple, and function syntax when using kind-projector

[travisbrown]

This is more a question than a proposal, I guess. I'm working on -Ykind-projector for Dotty, and I have a WIP implementation that I've confirmed can compile cats-core after #3207, #3193, and the change here (and one big Simulacrum rewrite).

I think it makes sense to try to minimize the scope of Dotty's kind-projector compatibility, and right now my implementation only handles the type-level λ[x => ...] syntax and * in ordinary type application (F[A, *, B, *, C]). This is a really tiny change—around 50 lines in the parser behind a single if (ctx.settings.YkindProjector.value). Adding support for * in tuple types, function types, and infix types would be a lot more invasive.

Maybe we should do it anyway, but for Cats at least only supporting * in type application isn't a very painful limitation—it only requires the changes here, and I think some of these changes (like A Ior * to Ior[A, *]) are readability improvements, anyway.

[djspiewak]

FWIW, I use function and tuple types with kind-projector all the time. Infix syntax, less so, though obviously it's the same code path. As a general stylistic rule, I think of Function1 and Tuple2 (and family) as being implementation details, and the eternally preferred way of referencing them should be their symbolic form.

[travisbrown]

@djspiewak Okay, thanks! I think "yes, we need * in function types and tuples" is a reasonable response, and I'm happy to implement it and advocate for it if people want it (I just don't really care, myself).

[codecov-io]

Codecov Report

Merging #3209 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3209   +/-   ##
=======================================
  Coverage   93.05%   93.05%           
=======================================
  Files         376      376           
  Lines        7412     7412           
  Branches      192      192           
=======================================
  Hits         6897     6897           
  Misses        515      515

Flag	Coverage Δ
#scala_version_212	93.38% <100%> (ø)	⬆️
#scala_version_213	92.81% <100%> (-0.02%)	⬇️

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/package.scala	88.88% <100%> (ø)	⬆️
core/src/main/scala/cats/data/Ior.scala	98.52% <100%> (ø)	⬆️
core/src/main/scala/cats/instances/tuple.scala	100% <100%> (ø)	⬆️
core/src/main/scala/cats/instances/function.scala	100% <100%> (ø)	⬆️
core/src/main/scala/cats/evidence/Is.scala	91.66% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b2711fb...95b0725. Read the comment docs.

[travisbrown]

See #3215 for a subset of these changes that I think are reasonable.