def parseA[_: P] = P("a")

val Parsed.Success(value, successIndex) = parse("a", parseA(_))

val Parsed.Success(value, successIndex) = parse("a", P("a"))

Error:(8, 61) overloaded method value P with alternatives:
  [T](t: fastparse.P[T])(implicit name: sourcecode.Name, implicit ctx: fastparse.P[_])fastparse.P[T] <and>
  => fastparse.ParsingRun.type
 cannot be applied to (String)
Error occurred in an application involving default arguments.
    val Parsed.Success(value, successIndex) = parse(source, P("a"))