CLIUtils/CLI11: ## Version 2.2: Option and Configuration Flexibility

New features include support for output of an empty vector, a summing option policy that can be applied more broadly, and an option to validate optional arguments to discriminate from positional arguments. A new validator to check for files on a default path is included to allow one or more default paths for configuration files or other file arguments. A number of bug fixes and code cleanup for various build configurations. Clean up of some error outputs and extension of existing capability to new types or situations.

There is a possible minor breaking change in behavior of certain types which wrapped an integer, such as std::atomic<int> or std::optional<int> when used in a flag. The default behavior is now as a single argument value vs. summing all the arguments. The default summing behavior is now restricted to pure integral types, int64_t, int, uint32_t, etc. Use the new sum multi option policy to revert to the older behavior. The summing behavior on wrapper types was not originally intended.

• Add MultiOptionPolicy::Sum and refactor the add_flag to fix a bug when using std::optional<bool> as type. #709
• Add support for an empty vector result in TOML and as a default string. #660
• Add .validate_optional_arguments() to support discriminating positional arguments from vector option arguments. #668
• Add CLI::FileOnDefaultPath to check for files on a specified default path. #698
• Change default value display in help messages from =XXXX to [XXXXX] to make it clearer. #666
• Modify the Range Validator to support additional types and clean up the error output. #690
• Bugfix: The trigger on parse modifier did not work on positional argument.s #713
• Bugfix: The single header file generation was missing custom namespace generation. #707
• Bugfix: Clean up File Error handling in the argument processing. #678
• Bugfix: Fix a stack overflow error if nameless commands had fallthrough. #665
• Bugfix: A subcommand callback could be executed multiple times if it was a member of an option group. #666
• Bugfix: Fix an issue with vectors of multi argument types where partial argument sets did not result in an error. #661
• Bugfix: Fix an issue with type the template matching on C++20 and add some CI builds for C++20. #663
• Bugfix: Fix typo in C++20 detection on MSVC. #706
• Bugfix: An issue where the detection of RTTI being disabled on certain MSVC platforms did not disable the use of dynamic cast calls. #666
• Bugfix: Resolve strict-overflow warning on some GCC compilers. #666
• Backend: Add additional tests concerning the use of aliases for option groups in config files. #666
• Build: Add support for testing in meson and cleanup symbolic link generation. #701, #697
• Build: Support building in WebAssembly. #679