A very simple example (using version 2.4.2) "fails" when called with -i "42 " (note the extra space at the end of the string), but works fine for e.g. -i " 42" (space before the integer) (and of course for the more natural way of calling it directly with an integer, e.g. -i 42

Is that a feature or a bug ? Knowing that e.g. std::stoi convert both " 42" and "42 " just fine, I'd call it a bug, but there might be something obvious I'm missing here ?

see e.g. https://godbolt.org/z/jad4MfsKP to run it

#include <iostream>
#include "CLI/CLI.hpp"

int main(int argc, char** argv)
{
  int value{0};

  CLI::App app{"basic cli example"};

  app.option_defaults()->always_capture_default();

  app.add_option("-i", value);

  try {
    app.parse(argc, argv);
  }
  catch (const CLI::ParseError& error) {
    std::cout << "CLI11 " << error.what() << "\n";
    return app.exit(error);
  }

  std::cout << "value=" << value << "\n";
}