public static List<Object> asArgumentList(Object args) {
    // there may be a better impl than this (maybe on some Groovy class)
    return asList(GroovyRuntimeUtil.asArgumentArray(args));
  }
Need to be careful because this converts null to an empty array.
For single arguments, new Object[] {arg} should therefore be used.
spock-core/src/main/java/org/spockframework/runtime/GroovyRuntimeUtil.java