Type of Scanner.rest different in Scala 2.13 and Scala 3

[OndrejSpanel]

Following code compiles fine with Scala 2, but does not compile with Scala 3:

import scala.util.parsing.combinator.lexical.Lexical

class ScalaLikeLexer extends Lexical {
  sealed abstract class ScalaLikeToken extends Token {
    def toSource = chars
  }

  @annotation.tailrec
  private def parseAllTokensLoop(s: Scanner, prevTokens: Seq[ScalaLikeToken]): Seq[ScalaLikeToken] = {
    if (s.atEnd) prevTokens.reverse // reversed, so that we can prepend (this is fast on Seq)
    else parseAllTokensLoop(s.rest, s.first.asInstanceOf[ScalaLikeToken] +: prevTokens)
  }
}

The error with Scala 3 is:

Found: scala.util.parsing.input.Reader[ScalaLikeLexer.this.Token]
Required: ScalaLikeLexer.this.Scanner

See https://scastie.scala-lang.org/bxUTWXp5SNeaXpgvqm24Mw

The reason is type of Scanner.rest is not specified in the source. The method is implemented using new Scanner(rest2), therefore the type is Scanner in Scala 2, while it is Reader[Token] in Scala 3 (see https://docs.scala-lang.org/scala3/guides/migration/incompat-type-inference.html)

Could the type be provided explicitly as Scanner, so that it is the same in all Scala versions? Other overridden members of Scanner could perhaps receive the same treatment.

If this seems acceptable, I can prepare the PR if desired.

[SethTisue]

Although I don't know the codebase well, the change certainly sounds plausible and desirable.

[Philippus]

Fixed in #480.