Add option to use underscore symbol _ instead of * to define anonymous type lambdas

[neko-kai]

== Dev Notes

This PR depends on the following scalac changes:

• Parse +_ and -_ in types as identifiers under -Xsource:3 to support Scala 3.2 placeholder syntax scala/scala#9605
• Differentiate Scala 2 and Scala 3 wildcard identifier names scala/scala#9614

To be widely applicable (without these changes the user would not have a way to write wildcards when using this mode and would have to write +_ and -_ with backticks – both would hinder cross-compilation with Scala 3)

The tests pass on all scalac versions (I've quoted +_, -_ for 2.10/2.11) compat. I've found the built-in test suite to be comprehensive, but small, so I've made sure that this change works by porting 7mind/izumi#1479 to the new syntax, with unquoted +_/-_ and, well, it works on my machine.

Fixes #120

== Description

The syntax roughly follows the proposed new syntax for wildcards and placeholders for Scala 3.2+ and is designed to allow cross-compilation of libraries between Scala 2 and Scala 3 while using the new Scala 3 syntax for both versions.

To enable this mode, add -P:kind-projector:underscore-placeholders to your scalac command-line. In sbt you may do this as follows:

ThisBuild / scalacOptions += "-P:kind-projector:underscore-placeholders"

This mode is designed to be used with scalac versions 2.12.14+ and 2.13.6+, these versions add an the ability to use ? as the existential type wildcard (scala/scala#9560), allowing to repurpose the underscore without losing the ability to write existential types. It is not advised that you use this mode with older versions of scalac or without -Xsource:3 flag, since you will lose the underscore syntax entirely.

Here are a few examples:

Tuple2[_, Double]        // equivalent to: type R[A] = Tuple2[A, Double]
Either[Int, +_]          // equivalent to: type R[+A] = Either[Int, A]
Function2[-_, Long, +_]  // equivalent to: type R[-A, +B] = Function2[A, Long, B]
EitherT[_[_], Int, _]    // equivalent to: type R[F[_], B] = EitherT[F, Int, B]

Examples with -Xsource:3's ?-wildcard:

Tuple2[_, ?]        // equivalent to: type R[A] = Tuple2[A, x] forSome { type x }
Either[?, +_]          // equivalent to: type R[+A] = Either[x, A] forSome { type x }
Function2[-_, ?, +_]  // equivalent to: type R[-A, +B] = Function2[A, x, B] forSome { type x }
EitherT[_[_], ?, _]    // equivalent to: type R[F[_], B] = EitherT[F, x, B] forSome { type x }

[neko-kai]

@larsrh @SethTisue The Scalac / Dotty PRs supporting this have been merged, so I think this could be merged, unless there are issues?

[larsrh]

Do we need the Scala 2.10 support still? Otherwise I'm inclined to drop this.

[neko-kai]

@larsrh Do you want me to drop 2.10 build as part of this PR? The case is required to compile on 2.10 but is not required otherwise.

[larsrh]

That would be great!

[neko-kai]

@larsrh Done in last 2 commits

[larsrh]

I can't push to your branch, but you'd have to run sbt githubWorkflowGenerate for CI to pass.

[larsrh]

LGTM, pending green build

[neko-kai]

@larsrh Build's green now 👍

I can't push to your branch

Odd, I don't think I changed anything to prohibit this, though I do not see an 'allow edits from maintainers' checkbox on this page either.

[larsrh]

Thanks!

[larsrh]

Since this is a new feature, not a breaking change, I think we can roll this as 0.12.1. I will update the README accordingly.

[neko-kai]

@larsrh Thanks! 🙏 For the release number, everywhere in the docs for the new feature I wrote version 0.13.0 and up (migration guide, dotty wildcards page, readme). It could be easier to release as 0.13 just to not update all of these...

[larsrh]

Fair enough, running that now.