OptionParser::Switch::PlacedArgument#parse

# File tmp/rubies/ruby-3.3.0/lib/optparse.rb, line 775
def parse(arg, argv, &error)
  if !(val = arg) and (argv.empty? or /\A-./ =~ (val = argv[0]))
    return nil, block, nil
  end
  opt = (val = parse_arg(val, &error))[1]
  val = conv_arg(*val)
  if opt and !arg
    argv.shift
  else
    val[0] = nil
  end
  val
end