Add support for Polytypes

[valencik]

This PR adds a new case for PolyType in TPrintImpl.

Hopefully this resolves:

• MatchError in TPrint macro expansion on PolyType #76
• MatchError on PolyType regression in 2.3.0 scalameta/mdoc#615

I've used @mrdziuban's scastie reproduction as a test case.

Disclaimer: I don't actually know what these PolyType's should look like.

We can test with the console via mill -i "pprint.jvm[2.13.4].console" and see what the compiler gives us:

scala> import scala.language.implicitConversions
     |
     | type Foo[+A] = Unit
     |
     | trait Bar[F[_], A]
     |
     | object Bar {
     |   implicit def anyToBar[A](a: A): Bar[Foo, A] = new Bar[Foo, A] {}
     |
     |   def apply[F[_], A](b: Bar[F, A]): Bar[F, A] = b
     | }
     |
     | def tprint[A](a: A)(implicit t: pprint.TPrint[A]) = t.render
import scala.language.implicitConversions
type Foo
trait Bar
object Bar
def tprint[A](a: A)(implicit t: pprint.TPrint[A]): fansi.Str

scala> tprint(Bar(1))
val res0: fansi.Str = Bar[Foo[A], Int]

scala> Bar(1)
val res1: Bar[[+A]Foo[A],Int] = Bar$$anon$1@794cb26b

As shown, the compiler prints Bar[[+A]Foo[A],Int] instead of our Bar[Foo[A], Int].
I feel like the [+A]Foo[A] is not usually how we'd write this, but happy to be corrected.

Update Mar 27th:

We now render Polytypes differently in Scala 2.12 versus 2.13:

Scala 2.13: Bar[[A]Foo[A], Int]
Scala 2.12: Bar[Foo, Int]

Disclaimer: I do not know if this is reasonable. It just so happens to work with minimal changes to PPrint.

[joroKr21]

I feel like the [+A]Foo[A] is not usually how we'd write this, but happy to be corrected.

Although this is not a type a user of Scala 2 can write, PolyTypes behave more or less like type lambdas and they can arise as result of compiler transformations (such as dealiasing) and they can also be created by macros.

[joroKr21]

Shouldn't it be

Suggested change

	assert(rendered.toString() == "Bar[Foo[A], Int]")
	assert(rendered.toString() == "Bar[[+A]Foo[A], Int]")

[valencik]

I've switched to Bar[[A]Foo[A], Int] for 2.13+, ditching the variance annotation as I don't think anything in PPrint uses the variance.

[joroKr21]

Shouldn't this be

Suggested change

	val params = typeParams.map(t => printArgSyms(t.asInstanceOf[TypeSymbol].typeParams)).mkString("")
	val params = printArgSyms(typeParams)

[valencik]

In scalajs 2.12.13 only, we are now failing with:

   utest.AssertionError: assert(rendered.toString() == "Bar[[A]Foo[A], Int]")
  rendered: fansi.Str = Bar[Foo, Int]

I unfortunately can't run the failing tests locally. When I try mill "pprint.js[2.12.13,0.6.33].test" I get:

...
[96/96] pprint.js[2.12.13,0.6.33].test.test 
Starting process: node
1 targets failed
pprint.js[2.12.13,0.6.33].test.test java.lang.RuntimeException: Failed to get framework
    mill.scalajslib.worker.ScalaJSWorkerImpl.$anonfun$getFramework$2(ScalaJSWorkerImpl.scala:141)
    scala.Option.getOrElse(Option.scala:201)
    mill.scalajslib.worker.ScalaJSWorkerImpl.getFramework(ScalaJSWorkerImpl.scala:141)
    mill.scalajslib.ScalaJSWorker.getFramework(ScalaJSWorkerApi.scala:68)
    mill.scalajslib.TestScalaJSModule.$anonfun$testTask$1(ScalaJSModule.scala:254)
    mill.define.Task$TraverseCtx.evaluate(Task.scala:379)

I do have node installed. And mill "pprint.js[2.13.4,1.5.1].test" works fine.
I'm not familiar with mill and don't know what else to try here.

[valencik]

Update: I was able to run the 2.12 tests locally with just mill "pprint.js[2.12.13,1.5.1].test" (not using 0.6.33)

[valencik]

We're now passing CI! Thanks for all the help @joroKr21 and @lolgab 🎉

I've updated the PR description to call out the different rendering in 2.13 vs 2.12.
This PR is now ready for review.

[lolgab]

LGTM! 👍
Thank you for the contribution!