GCC warns about unused parameters in ArgumentParser class

[ericonr]

Seems to be caused by b5fb663

[1/2] Compiling C++ object decode-reg.p/decode-reg.cc.o
In file included from ../decode-reg.cc:16:
../subprojects/argparse-2.3/include/argparse/argparse.hpp: In instantiation of ‘argparse::ArgumentParser::ArgumentParser(std::string, std::string, argparse::default_arguments)::<lambda(const auto:33&)> [with auto:33 = std::__cxx11::basic_string<char>]’:
/usr/include/c++/9/type_traits:2330:26:   required by substitution of ‘template<class _Fn, class ... _Args> static std::__result_of_success<decltype (declval<_Fn>()((declval<_Args>)()...)), std::__invoke_other> std::__result_of_other_impl::_S_test(int) [with _Fn = argparse::ArgumentParser::ArgumentParser(std::string, std::string, argparse::default_arguments)::<lambda(const auto:33&)>; _Args = {const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >}]’
/usr/include/c++/9/type_traits:2341:55:   required from ‘struct std::__result_of_impl<false, false, argparse::ArgumentParser::ArgumentParser(std::string, std::string, argparse::default_arguments)::<lambda(const auto:33&)>, const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >’
/usr/include/c++/9/type_traits:2781:12:   recursively required by substitution of ‘template<class _Result, class _Ret> struct std::__is_invocable_impl<_Result, _Ret, true, std::__void_t<typename _Result::type> > [with _Result = std::__invoke_result<argparse::ArgumentParser::ArgumentParser(std::string, std::string, argparse::default_arguments)::<lambda(const auto:33&)>, const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >; _Ret = void]’
/usr/include/c++/9/type_traits:2781:12:   required from ‘struct std::is_invocable<argparse::ArgumentParser::ArgumentParser(std::string, std::string, argparse::default_arguments)::<lambda(const auto:33&)>, const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >’
/usr/include/c++/9/type_traits:2818:27:   required from ‘constexpr const bool std::is_invocable_v<argparse::ArgumentParser::ArgumentParser(std::string, std::string, argparse::default_arguments)::<lambda(const auto:33&)>, const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >’
../subprojects/argparse-2.3/include/argparse/argparse.hpp:391:32:   required by substitution of ‘template<class F, class ... Args> std::enable_if_t<is_invocable_v<F, Args ..., const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, argparse::Argument&> argparse::Argument::action(F&&, Args&& ...) [with F = argparse::ArgumentParser::ArgumentParser(std::string, std::string, argparse::default_arguments)::<lambda(const auto:33&)>; Args = {}]’
../subprojects/argparse-2.3/include/argparse/argparse.hpp:861:12:   required from here
../subprojects/argparse-2.3/include/argparse/argparse.hpp:858:35: warning: unused parameter ‘unused’ [-Wunused-parameter]
  858 |           .action([&](const auto &unused) {
      |                       ~~~~~~~~~~~~^~~~~~
../subprojects/argparse-2.3/include/argparse/argparse.hpp: In instantiation of ‘argparse::ArgumentParser::ArgumentParser(std::string, std::string, argparse::default_arguments)::<lambda(const auto:34&)> [with auto:34 = std::__cxx11::basic_string<char>]’:
/usr/include/c++/9/type_traits:2330:26:   required by substitution of ‘template<class _Fn, class ... _Args> static std::__result_of_success<decltype (declval<_Fn>()((declval<_Args>)()...)), std::__invoke_other> std::__result_of_other_impl::_S_test(int) [with _Fn = argparse::ArgumentParser::ArgumentParser(std::string, std::string, argparse::default_arguments)::<lambda(const auto:34&)>; _Args = {const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >}]’
/usr/include/c++/9/type_traits:2341:55:   required from ‘struct std::__result_of_impl<false, false, argparse::ArgumentParser::ArgumentParser(std::string, std::string, argparse::default_arguments)::<lambda(const auto:34&)>, const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >’
/usr/include/c++/9/type_traits:2781:12:   recursively required by substitution of ‘template<class _Result, class _Ret> struct std::__is_invocable_impl<_Result, _Ret, true, std::__void_t<typename _Result::type> > [with _Result = std::__invoke_result<argparse::ArgumentParser::ArgumentParser(std::string, std::string, argparse::default_arguments)::<lambda(const auto:34&)>, const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >; _Ret = void]’
/usr/include/c++/9/type_traits:2781:12:   required from ‘struct std::is_invocable<argparse::ArgumentParser::ArgumentParser(std::string, std::string, argparse::default_arguments)::<lambda(const auto:34&)>, const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >’
/usr/include/c++/9/type_traits:2818:27:   required from ‘constexpr const bool std::is_invocable_v<argparse::ArgumentParser::ArgumentParser(std::string, std::string, argparse::default_arguments)::<lambda(const auto:34&)>, const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >’
../subprojects/argparse-2.3/include/argparse/argparse.hpp:391:32:   required by substitution of ‘template<class F, class ... Args> std::enable_if_t<is_invocable_v<F, Args ..., const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, argparse::Argument&> argparse::Argument::action(F&&, Args&& ...) [with F = argparse::ArgumentParser::ArgumentParser(std::string, std::string, argparse::default_arguments)::<lambda(const auto:34&)>; Args = {}]’
../subprojects/argparse-2.3/include/argparse/argparse.hpp:872:12:   required from here
../subprojects/argparse-2.3/include/argparse/argparse.hpp:869:35: warning: unused parameter ‘unused’ [-Wunused-parameter]
  869 |           .action([&](const auto &unused) {
      |                       ~~~~~~~~~~~~^~~~~~
../subprojects/argparse-2.3/include/argparse/argparse.hpp: In instantiation of ‘argparse::Argument::Argument(std::array<std::basic_string_view<char>, N>&&, std::index_sequence<__indices ...>) [with long unsigned int N = 2; long unsigned int ...I = {0, 1}; std::index_sequence<__indices ...> = std::integer_sequence<long unsigned int, 0, 1>]’:
../subprojects/argparse-2.3/include/argparse/argparse.hpp:365:61:   required from ‘argparse::Argument::Argument(std::array<std::basic_string_view<char>, N>&&) [with long unsigned int N = 2]’
/usr/include/c++/9/ext/new_allocator.h:146:4:   required from ‘void __gnu_cxx::new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = argparse::Argument; _Args = {std::array<std::basic_string_view<char, std::char_traits<char> >, 2>}; _Tp = std::_List_node<argparse::Argument>]’
/usr/include/c++/9/bits/alloc_traits.h:483:4:   required from ‘static void std::allocator_traits<std::allocator<_Tp1> >::construct(std::allocator_traits<std::allocator<_Tp1> >::allocator_type&, _Up*, _Args&& ...) [with _Up = argparse::Argument; _Args = {std::array<std::basic_string_view<char, std::char_traits<char> >, 2>}; _Tp = std::_List_node<argparse::Argument>; std::allocator_traits<std::allocator<_Tp1> >::allocator_type = std::allocator<std::_List_node<argparse::Argument> >]’
/usr/include/c++/9/bits/stl_list.h:633:33:   required from ‘std::__cxx11::list<_Tp, _Alloc>::_Node* std::__cxx11::list<_Tp, _Alloc>::_M_create_node(_Args&& ...) [with _Args = {std::array<std::basic_string_view<char, std::char_traits<char> >, 2>}; _Tp = argparse::Argument; _Alloc = std::allocator<argparse::Argument>; std::__cxx11::list<_Tp, _Alloc>::_Node = std::_List_node<argparse::Argument>]’
/usr/include/c++/9/bits/list.tcc:92:9:   required from ‘std::__cxx11::list<_Tp, _Alloc>::iterator std::__cxx11::list<_Tp, _Alloc>::emplace(std::__cxx11::list<_Tp, _Alloc>::const_iterator, _Args&& ...) [with _Args = {std::array<std::basic_string_view<char, std::char_traits<char> >, 2>}; _Tp = argparse::Argument; _Alloc = std::allocator<argparse::Argument>; std::__cxx11::list<_Tp, _Alloc>::iterator = std::_List_iterator<argparse::Argument>; std::__cxx11::list<_Tp, _Alloc>::const_iterator = std::_List_const_iterator<argparse::Argument>]’
../subprojects/argparse-2.3/include/argparse/argparse.hpp:913:10:   required from ‘argparse::Argument& argparse::ArgumentParser::add_argument(Targs ...) [with Targs = {const char*, const char*}]’
../subprojects/argparse-2.3/include/argparse/argparse.hpp:857:34:   required from here
../subprojects/argparse-2.3/include/argparse/argparse.hpp:352:47: warning: unused parameter ‘unused’ [-Wunused-parameter]
  352 |                     std::index_sequence<I...> unused)
      |                     ~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~

We can add the [maybe_unused] attribute from C++17, but I think removing the lint makes more sense, since leaving the parameter name blank is a C++ feature that it makes sense to take advantage of in this case, IMO.

[ericonr]

From https://google.github.io/styleguide/cppguide.html#Function_Declarations_and_Definitions , which explains the rationale for this lint:

A parameter name may be omitted only if the parameter is not used in the function's definition.

It seems clang-tidy v12 did not implement the lint correctly, but v13 did, as noticed in the commit message (but was attributed to a bug in v13 instead of being correct):

Note that clang-tidy v12 appears to detect unnamed parameters in lambdas,
while clang-tidy v13 does not.

[skrobinson]

I like documenting that the parameter is intentionally unused by the function, rather than just leaving it out. How does the following pattern work for you?

-          .action([&](const auto &unused) {
+          .action([&](const auto &/*unused*/) {

This passes with clang-tidy v13 and the current config file. For now, v12 is not linting PR.

[ericonr]

I like that pattern!