Can`t parse "--help" argument about args

[Dmry]

Everything works without fault on my Linux machine, so this might be windows specific.

[marvin-Yu]

I debugged the case to find the exact cause. Finally I found that this lambda expression can`t capture the variables correctly in windows.

 [&, this](auto&& attribute) -> decltype(attribute(x, *this, arg), void()) { attribute(x, *this, arg); }

template<class T, class... Ts>
void parse(T&& x, Ts&&... xs)
{
    argument arg;
    arg.write_value = [&x](const std::string& s) { args::write_value_to(x, s); };
    arg.type = args::get_argument_type(x);
    arg.metavar = args::type_to_help(x);
    args::each_arg(args::overload(
        [&](const std::string& name) { arg.flags.push_back(name); },
        [&, this](auto&& attribute) -> decltype(attribute(x, *this, arg), void()) { attribute(x, *this, arg); }
    ), std::forward<Ts>(xs)...);
    this->add(std::move(arg));
}

By the way, I can`t compile the "args.hpp" file directly at vs2019.
vs2019 compile output show as below.

1>------ Build started: Project: TestArg, Configuration: Debug x64 ------
1>TestArg.cpp
1>C:\Users\weifeiyu\source\repos\TestArg\args.hpp(344,1): error C3861: '__this': identifier not found
...

[pfultz2]

I think this is a problem with handling expression SFINAE in MSVC. I've never tested this on windows. If there is some workarounds to make this work on MSVC, please let me know.

[marvin-Yu]

I'm newbie here of c++, so the root cause I can't figure out. In my test, this issue just happen in the "--help" parser. Cause of the project limit, I have to do some ugly modification, but work.
In order to avoid compile error, I use local variable to instead of this pointer.

template<class T, class... Ts>                                                                              |template<class T, class... Ts>
void parse(T&& x, Ts&&... xs)                                                                               |void parse(T&& x, Ts&&... xs)
{                                                                                                           |{
    context* cxt = this;                                                                                    |-------------------------------------------------------------------------------------------------------------------------------------
    argument arg;                                                                                           |    argument arg;
    arg.write_value = [&x](const std::string& s) { args::write_value_to(x, s); };                           |    arg.write_value = [&x](const std::string& s) {args::write_value_to(x, s); };
    arg.type = args::get_argument_type(x);                                                                  |    arg.type = args::get_argument_type(x);
    arg.metavar = args::type_to_help(x);                                                                    |    arg.metavar = args::type_to_help(x);
    args::each_arg(args::overload(                                                                          |    args::each_arg(args::overload(
        [&](const std::string& name) { arg.flags.push_back(name); },                                        |        [&, this](const std::string& name) { arg.flags.push_back(name); },
        [&, cxt](auto&& attribute) -> decltype(attribute(x, *cxt, arg), void()) { attribute(x, *cxt, arg); }|        [&, this](auto&& attribute) -> decltype(attribute(x, *this, arg), void()) { attribute(x, *this, arg); }
    ), std::forward<Ts>(xs)...);                                                                            |    ), std::forward<Ts>(xs)...);
    this->add(std::move(arg));                                                                              |    this->add(std::move(arg));
}                                                                                                           |}
          

Then, Using the 'ctx' variable captured by copy to support show "--help" information.

template<class... Ts, class T>                                                    |  template<class... Ts, class T>
context<T&, Ts...> build_context(T& cmd)                                          |  context<T&, Ts...> build_context(T& cmd)
{                                                                                 |  {
    context<T&, Ts...> ctx;                                                       |      context<T&, Ts...> ctx;
    args::assign_subcommands(rank<1>{}, ctx, cmd);                                |      args::assign_subcommands(rank<1>{}, ctx, cmd);
                                                                                  |      ctx.parse(nullptr, "--help", "-h", args::help("Show help."),
----------------------------------------------------------------------------------|          args::eager_callback([](std::nullptr_t, const auto& c, const argument&)
----------------------------------------------------------------------------------|      {
----------------------------------------------------------------------------------|          c.show_help(get_name<T>(), get_help<T>(), get_options_metavar<T>());
----------------------------------------------------------------------------------|      }));
    args::try_parse(rank<1>{}, cmd, [&](auto&&... xs)                             |      args::try_parse(rank<1>{}, cmd, [&](auto&&... xs)
    {                                                                             |      {
        ctx.parse(std::forward<decltype(xs)>(xs)...);                             |          ctx.parse(std::forward<decltype(xs)>(xs)...);
    });                                                                           |      });
    ctx.parse(nullptr, "--help", "-h", args::help("Show help."),                  |  -------------------------------------------------------------------------------
        args::eager_callback([ctx](std::nullptr_t, const auto& c, const argument&)|  -------------------------------------------------------------------------------
    {                                                                             |  -------------------------------------------------------------------------------
        ctx.show_help(get_name<T>(), get_help<T>(), get_options_metavar<T>());    |  -------------------------------------------------------------------------------
    }));                                                                          |  -------------------------------------------------------------------------------
    return ctx;                                                                   |      return ctx;
}                                                                                 |  }
  

Hope someone can solve this problem really.