avoid underscore as identifier for Scala 2.13.x ?

[xuwei-k]

• Disallow _ as an identifier scala/bug#10384
• Import selector respects backquotes [ci: last-only] scala/scala#6218
• https://github.com/lihaoyi/fastparse/blob/d2298abdc5d467012fb1520491ca9ca5885a0096/scalaparse/shared/src/main/scala/scalaparse/Core.scala#L36
• https://scala-ci.typesafe.com/job/scala-2.13.x-integrate-community-build/1703/consoleFull

[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Core.scala:83:17: not found: value _
[fastparse] [error]   val `_*` = P( `_` ~ `*` )
[fastparse] [error]                 ^
[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Core.scala:83:17: not found: value _
[fastparse] [error]   val `_*` = P( `_` ~ `*` )
[fastparse] [error]                 ^
[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Core.scala:103:61: not found: value _
[fastparse] [error]   val PostDotCheck: P0 = P( WL ~ !(`super` | `this` | "{" | `_` | `type`) )
[fastparse] [error]                                                             ^
[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Core.scala:103:61: not found: value _
[fastparse] [error]   val PostDotCheck: P0 = P( WL ~ !(`super` | `this` | "{" | `_` | `type`) )
[fastparse] [error]                                                             ^
[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Exprs.scala:14:33: not found: value _
[fastparse] [error]     val Selector: P0 = P( (Id | `_`) ~ (`=>` ~/ (Id | `_`)).? )
[fastparse] [error]                                 ^
[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Exprs.scala:14:55: not found: value _
[fastparse] [error]     val Selector: P0 = P( (Id | `_`) ~ (`=>` ~/ (Id | `_`)).? )
[fastparse] [error]                                                       ^
[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Exprs.scala:14:33: not found: value _
[fastparse] [error]     val Selector: P0 = P( (Id | `_`) ~ (`=>` ~/ (Id | `_`)).? )
[fastparse] [error]                                 ^
[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Exprs.scala:14:55: not found: value _
[fastparse] [error]     val Selector: P0 = P( (Id | `_`) ~ (`=>` ~/ (Id | `_`)).? )
[fastparse] [error]                                                       ^
[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Exprs.scala:16:49: not found: value _
[fastparse] [error]     val ImportExpr: P0 = P( StableId ~ ("." ~/ (`_` | Selectors)).? )
[fastparse] [error]                                                 ^
[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Exprs.scala:16:49: not found: value _
[fastparse] [error]     val ImportExpr: P0 = P( StableId ~ ("." ~/ (`_` | Selectors)).? )
[fastparse] [error]                                                 ^
[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Exprs.scala:74:50: not found: value _
[fastparse] [error]       val ImplicitLambda = P( `implicit` ~ (Id | `_`) ~ (`:` ~ InfixType).? ~ `=>` ~ LambdaRhs.? )
[fastparse] [error]                                                  ^
[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Exprs.scala:74:50: not found: value _
[fastparse] [error]       val ImplicitLambda = P( `implicit` ~ (Id | `_`) ~ (`:` ~ InfixType).? ~ `=>` ~ LambdaRhs.? )
[fastparse] [error]                                                  ^
[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Exprs.scala:88:98: not found: value _
[fastparse] [error]     val ExprSuffix = P( (WL ~ "." ~/ Id | WL ~ TypeArgs | NoSemis ~ ArgList).repX ~~ (NoSemis  ~ `_`).? )
[fastparse] [error]                                                                                                  ^
[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Exprs.scala:88:98: not found: value _
[fastparse] [error]     val ExprSuffix = P( (WL ~ "." ~/ Id | WL ~ TypeArgs | NoSemis ~ ArgList).repX ~~ (NoSemis  ~ `_`).? )
[fastparse] [error]                                                                                                  ^
[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Exprs.scala:104:63: not found: value _
[fastparse] [error]       P( XmlExpr | New | BlockExpr | ExprLiteral | StableId | `_` | Parened )
[fastparse] [error]                                                               ^
[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Exprs.scala:111:21: not found: value _
[fastparse] [error]     val Thingy = P( `_` ~ (`:` ~/ TypePat).? ~ !("*" ~~ !syntax.Basic.OpChar) )
[fastparse] [error]                     ^
[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Exprs.scala:104:63: not found: value _
[fastparse] [error]       P( XmlExpr | New | BlockExpr | ExprLiteral | StableId | `_` | Parened )
[fastparse] [error]                                                               ^
[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Exprs.scala:111:21: not found: value _
[fastparse] [error]     val Thingy = P( `_` ~ (`:` ~/ TypePat).? ~ !("*" ~~ !syntax.Basic.OpChar) )
[fastparse] [error]                     ^
[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Exprs.scala:117:76: not found: value _
[fastparse] [error]   val BlockLambdaHead: P0 = P( "(" ~ BlockLambdaHead ~ ")" | `this` | Id | `_` )
[fastparse] [error]                                                                            ^
[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Exprs.scala:117:76: not found: value _
[fastparse] [error]   val BlockLambdaHead: P0 = P( "(" ~ BlockLambdaHead ~ ")" | `this` | Id | `_` )
[fastparse] [error]                                                                            ^
[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Exprs.scala:136:25: not found: value _
[fastparse] [error]   val TypePattern = P( (`_` | BacktickId | VarId) ~ `:` ~ TypePat )
[fastparse] [error]                         ^
[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Exprs.scala:140:28: not found: value _
[fastparse] [error]     val Binding = P( (Id | `_`) ~ `@` )
[fastparse] [error]                            ^
[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Exprs.scala:136:25: not found: value _
[fastparse] [error]   val TypePattern = P( (`_` | BacktickId | VarId) ~ `:` ~ TypePat )
[fastparse] [error]                         ^
[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Exprs.scala:140:28: not found: value _
[fastparse] [error]     val Binding = P( (Id | `_`) ~ `@` )
[fastparse] [error]                            ^
[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Types.scala:44:89: not found: value _
[fastparse] [error]     val BasicType = P( TupleType | Literals.Expr.Literal | TypeId ~ ("." ~ `type`).?  | `_` )
[fastparse] [error]                                                                                         ^
[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Types.scala:44:89: not found: value _
[fastparse] [error]     val BasicType = P( TupleType | Literals.Expr.Literal | TypeId ~ ("." ~ `type`).?  | `_` )
[fastparse] [error]                                                                                         ^
[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Types.scala:62:13: not found: value _
[fastparse] [error]     P((Id | `_`) ~ TypeArgList.? ~ TypeBounds ~ CtxBounds)
[fastparse] [error]             ^
[fastparse] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.16/project-builds/fastparse-2dafd52f84a227c6da3fd1a716b2b02576a4f25e/scalaparse/shared/src/main/scala/scalaparse/Types.scala:62:13: not found: value _
[fastparse] [error]     P((Id | `_`) ~ TypeArgList.? ~ TypeBounds ~ CtxBounds)
[fastparse] [error]             ^
[fastparse] [error] 14 errors found
[fastparse] [info] No documentation generated with unsuccessful compiler run
[fastparse] [error] 14 errors found
[fastparse] [error] (scalaparseJVM / Compile / compileIncremental) Compilation failed
[fastparse] [error] (scalaparseJVM / Compile / doc) Scaladoc generation failed

[lihaoyi]

I'm happy to rename it if necessary, but aren't the backticks meant to allow usage of special characters and keywords in identifiers?