GCC: Wmaybe-uninitialized in indexing_suite_detail

Commencez par include/boost/python/suite/indexing/detail/indexing_suite_detail.hpp à la ligne 678 et reproduisez l’avertissement à l’aide de l’exemple minimal et de la commande GCC indiqués. Suivez le chemin indexing_suite_detail jusqu’à container_utils.hpp pour comprendre la lecture non initialisée signalée. Le travail est terminé lorsque l’exemple se compile avec GCC 15.2.1 sans cet avertissement, tout en préservant le comportement d’indexation des vecteurs.

Recent GCC versions (tested on g++ (GCC) 15.2.1 20260123 (Red Hat 15.2.1-7) on Fedora 43 with Boost 1.83, but should not differ from newer Boost versions), when compiling this minimal working example:

#include <boost/python.hpp>
#include <boost/python/suite/indexing/vector_indexing_suite.hpp>
namespace py = boost::python;

BOOST_PYTHON_MODULE(Types) {
    py::class_<std::vector<std::string>>("StringVector").def(py::vector_indexing_suite<std::vector<std::string>>());
}

with this command line: g++ -shared -fPIC Types.cxx -Wextra -O2 -std=c++17 -I/usr/include/python3.14 -I/usr/include/boost - lboost_python314 -lpython3.14

throw Wmaybe-uninitialized:

In file included from /usr/include/c++/15/string:56,
                 from /usr/include/boost/assert/source_location.hpp:15,
                 from /usr/include/boost/exception/exception.hpp:9,
                 from /usr/include/boost/throw_exception.hpp:21,
                 from /usr/include/boost/function/detail/prologue.hpp:16,
                 from /usr/include/boost/function/function_template.hpp:13,
                 from /usr/include/boost/function/detail/maybe_include.hpp:15,
                 from /usr/include/boost/function/function0.hpp:11,
                 from /usr/include/boost/python/errors.hpp:13,
                 from /usr/include/boost/python/handle.hpp:11,
                 from /usr/include/boost/python/args_fwd.hpp:10,
                 from /usr/include/boost/python/args.hpp:10,
                 from /usr/include/boost/python.hpp:11,
                 from Types.cxx:1:
In member function ‘std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size() const [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’,
    inlined from ‘std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::length() const [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’ at /usr/include/c++/15/bits/basic_string.h:1176:20,
    inlined from ‘std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’ at /usr/include/c++/15/bits/basic_string.h:617:20,
    inlined from ‘void std::__new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = std::__cxx11::basic_string<char>; _Args = {const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&}; _Tp = std::__cxx11::basic_string<char>]’ at /usr/include/c++/15/bits/new_allocator.h:191:4,
    inlined from ‘static void std::allocator_traits<std::allocator<_Tp1> >::construct(allocator_type&, _Up*, _Args&& ...) [with _Up = std::__cxx11::basic_string<char>; _Args = {const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&}; _Tp = std::__cxx11::basic_string<char>]’ at /usr/include/c++/15/bits/alloc_traits.h:674:17,
    inlined from ‘void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::__cxx11::basic_string<char>; _Alloc = std::allocator<std::__cxx11::basic_string<char> >]’ at /usr/include/c++/15/bits/stl_vector.h:1421:30,
    inlined from ‘void boost::python::container_utils::extend_container(Container&, boost::python::api::object) [with Container = std::vector<std::__cxx11::basic_string<char> >]’ at /usr/include/boost/python/suite/indexing/container_utils.hpp:44:40:
/usr/include/c++/15/bits/basic_string.h:1165:19: warning: ‘*(const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*)((char*)&x + offsetof(boost::python::extract<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >,boost::python::extract<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::<unnamed>.boost::python::converter::extract_rvalue<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::m_data.boost::python::converter::rvalue_from_python_data<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::<unnamed>.boost::python::converter::rvalue_from_python_storage<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::storage)).std::__cxx11::basic_string<char>::_M_string_length’ may be used uninitialized [-Wmaybe-uninitialized]
 1165 |         size_type __sz = _M_string_length;
      |                   ^~~~

This can be traced down to originating from this line in indexing_suite.hpp: https://github.com/boostorg/python/blob/105d64e8895ab893e5ac9e0380647d216c2cb8d9/include/boost/python/suite/indexing/detail/indexing_suite_detail.hpp#L678

Since this was not touched since Boost 1.83, I believe this to still be relevant. This seems unintentional and due to our project treating warnings as errors, for us this leads to a compile error.