Combination .mend(???).within(???) not working inside split block

[NPCRUS]

trait Defooler[T] {
    def defool(a: String): Option[String]
  }
  object Defooler extends Derivation[Defooler] {
    inline def join[DerivationType <: Product](using ProductReflection[DerivationType]): Defooler[DerivationType] = ???

    inline def split[DerivationType](using SumReflection[DerivationType]): Defooler[DerivationType] = str =>
      mend {
        case e: VariantError =>
          Left(e.getMessage)
      }.within {
        delegate(str)([VariantType <: DerivationType] => defooler =>
          defooler.defool(str)  
        )
      }
  }

produces:

value within is not a member of Any.
An extension method was tried, but could not be fully constructed:

    contingency.within()

    failed with:

        value within: <overloaded contingency.within> does not take parameters

at .within