Accessing backqouted underscore does not parse but compiles fine in Scala 2.13.x

[romanowski]

Compiler version

I tested with3.1.1 but it probably applies to all Scala 3 / dotty versions

Minimized code

//> using lib "com.lihaoyi:scalaparse_2.13:2.3.3"

import fastparse._

class U extends App {
  def a[T: P] = scalaparse.Scala.`_` 
                                                    // ^ fails here with: wildcard invalid as backquoted identifier
}

Output

Compiles on Scala 2.13.8, fails on 3.1.1 with:

[error] ./underscore.scala:6:34: wildcard invalid as backquoted identifier
[error]   def a[T: P] = scalaparse.Scala.`_`
[error]                                  ^
Error compiling project (Scala 3.1.1, JVM)

Expectation

I know that_ is not the best name of the method, however, it exists and is used e.g. in Scala CLI.

[som-snytt]

This has a long history on Scala 2, where support was for what version of Java allowed underscore.

The dotty behavior is noted at scala/bug#10384 (comment)

The import behavior was supported at scala/scala#6218

In a definition scala/scala#7669

I wonder if you're allowed to have a class named underscore.

[odersky]

_ has a special meaning in the compiler. It should not be treated as a regular identifier. The situations where we still treat it as an identifier are probably all bugs or at least in some form problematic.

What is the use case in scala-cli?

[Kordyjan]

The problem here seems to be the fact that Scala 2.13 libraries were using _ as a function name and now they are unusable from Scala 3. Here is the one that caused the problem in scala-cli.

[odersky]

The fastparse code treats _ like a keyword. Problem is, the tree does not reflect this. _ is treated as Ident("_"). So backquoting is very tricky. Maybe we can make it work in only 3.0-migration mode? Would that work for scala-cli?