When an optional option with `choices` is used with `nargs(1)`, if the option appears last, an empty argument is accepted when it shouldn't.

[palapapa]

#include <argparse/argparse.hpp>
#include <iostream>
#include <string>

int main() {
  argparse::ArgumentParser program("test");

  program.add_argument("--color")
      .choices("red", "green")
      .nargs(1)
      .action([](const std::string &value) {
        std::cout << "action called with size="
                  << value.size() << '\n';
        return value;
      });

  try {
    program.parse_args({"test", "--color"});
    std::cout << "parse succeeded\n";
  } catch (const std::exception &error) {
    std::cout << "caught: " << error.what() << '\n';
  }
}

I intend the option --color to have a default value when it isn't specified, but when it is, it must have an argument. So either you don't specify --color at all, or you have to specify --color red or --color green. The example program prints:

action called with size=0
parse succeeded

when it should have failed.