Parenthesize lambda parameters with type annotations

[travisbrown]

Dotty warns on these and Scala 2 doesn't care, so it seems like we might as well change them now. I personally agree with Dotty and think they look better with the parentheses, anyway.

Dotty says it'll rewrite these with -rewrite but I couldn't get it to work so I just used a quick custom Scalafix rule:

import scalafix.v1.{Patch, SyntacticDocument, SyntacticRule}
import scala.meta.{Term, Token}

class TypedLambdaParamRewrite extends SyntacticRule("TypedLambdaParamRewrite") {
  override def fix(implicit doc: SyntacticDocument): Patch = Patch.fromIterable(
    doc.tree.collect {
      case Term.Function(List(param @ Term.Param(_, _, Some(_), None)), _) =>
        param.tokens.headOption match {
          case Some(Token.LeftParen()) => Patch.empty
          case _ => Patch.replaceTree(param, s"($param)")
        }
    }
  )
}

[codecov-io]

Codecov Report

Merging #3182 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3182   +/-   ##
=======================================
  Coverage   92.99%   92.99%           
=======================================
  Files         376      376           
  Lines        7381     7381           
  Branches      195      201    +6     
=======================================
  Hits         6864     6864           
  Misses        517      517

Flag	Coverage Δ
#scala_version_212	93.31% <100%> (-0.02%)	⬇️
#scala_version_213	90.59% <100%> (+0.01%)	⬆️

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/IndexedStateT.scala	90% <ø> (ø)	⬆️
core/src/main/scala/cats/Eval.scala	98.83% <ø> (ø)	⬆️
core/src/main/scala/cats/data/Func.scala	96.42% <100%> (ø)	⬆️
core/src/main/scala/cats/Representable.scala	100% <100%> (ø)	⬆️
core/src/main/scala/cats/data/Kleisli.scala	90.82% <100%> (ø)	⬆️
core/src/main/scala/cats/data/ContT.scala	100% <100%> (ø)	⬆️
core/src/main/scala/cats/Composed.scala	96.87% <100%> (ø)	⬆️

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[rossabaker]

I like it.

[kailuowang]

Thanks for sharing the scalafix code as well.