template <typename jsType, typename jsParamType, typename ParamPrivateType = jsParamType::PrivateType, void(PrivateType::*Method)(const ParamPrivateType&)>
    static bool SetByRefMethod(JSContext *cx, unsigned argc, JS::Value *vp)
{
...
}

 template <typename jsType, typename jsParamType, typename ParamPrivateType = jsParamType::PrivateType, void(PrivateType::*Method)(      ParamPrivateType&)>
    static bool SetByRefMethod(JSContext *cx, unsigned argc, JS::Value *vp)
{
...
}