GCC 14/15 `-Wmaybe-uninitialized` false positive in `tuple_rule.hpp`

[ashtum]

grammar::parse with authority_rule triggers -Wmaybe-uninitialized on GCC 14/15 at -O3.

Reproducer:

#include <boost/url/grammar/parse.hpp>
#include <boost/url/rfc/authority_rule.hpp>

int main()
{
    (void)boost::urls::grammar::parse(
        "", boost::urls::authority_rule);
}

Compile command:

g++ -std=c++20 -O3 -Wall -Wextra -Werror -c mre.cpp

Warning:

In file included from /workspace/boost/boost/url/grammar/tuple_rule.hpp:237,
                 from /workspace/boost/boost/url/rfc/impl/authority_rule.hpp:18,
                 from /workspace/boost/boost/url/rfc/authority_rule.hpp:72,
                 from mre.cpp:2:
In member function ‘constexpr void boost::urls::grammar::detail::parse_sequence<false, R0, Rn ...>::apply(const char*&, const char*, boost::mp11::mp_size_t<I>&, boost::mp11::mp_size_t<Iv>&, const boost::mp11::mp_true&) [with long unsigned int Ir = 1; long unsigned int Iv = 1; R0 = boost::urls::detail::ipvfuture_rule_t; Rn = {boost::urls::grammar::implementation_defined::squelch_rule_t<boost::urls::grammar::implementation_defined::ch_delim_rule>}]’,
    inlined from ‘constexpr typename std::enable_if<(Ir < (1 + sizeof... (Rn)))>::type boost::urls::grammar::detail::parse_sequence<false, R0, Rn ...>::apply(const char*&, const char*, boost::mp11::mp_size_t<I>&, boost::mp11::mp_size_t<Iv>&) [with long unsigned int Ir = 1; long unsigned int Iv = 1; R0 = boost::urls::detail::ipvfuture_rule_t; Rn = {boost::urls::grammar::implementation_defined::squelch_rule_t<boost::urls::grammar::implementation_defined::ch_delim_rule>}]’ at /workspace/boost/boost/url/grammar/impl/tuple_rule.hpp:265:14,
    inlined from ‘constexpr void boost::urls::grammar::detail::parse_sequence<false, R0, Rn ...>::apply(const char*&, const char*, boost::mp11::mp_size_t<I>&, boost::mp11::mp_size_t<Iv>&, const boost::mp11::mp_false&) [with long unsigned int Ir = 0; long unsigned int Iv = 0; R0 = boost::urls::detail::ipvfuture_rule_t; Rn = {boost::urls::grammar::implementation_defined::squelch_rule_t<boost::urls::grammar::implementation_defined::ch_delim_rule>}]’ at /workspace/boost/boost/url/grammar/impl/tuple_rule.hpp:247:14,
    inlined from ‘constexpr typename std::enable_if<(Ir < (1 + sizeof... (Rn)))>::type boost::urls::grammar::detail::parse_sequence<false, R0, Rn ...>::apply(const char*&, const char*, boost::mp11::mp_size_t<I>&, boost::mp11::mp_size_t<Iv>&) [with long unsigned int Ir = 0; long unsigned int Iv = 0; R0 = boost::urls::detail::ipvfuture_rule_t; Rn = {boost::urls::grammar::implementation_defined::squelch_rule_t<boost::urls::grammar::implementation_defined::ch_delim_rule>}]’ at /workspace/boost/boost/url/grammar/impl/tuple_rule.hpp:265:14,
    inlined from ‘constexpr boost::system::result<typename boost::mp11::detail::mp_eval_if_c_impl<boost::urls::grammar::implementation_defined::tuple_rule_t<R0, Rn>::IsList, typename boost::mp11::detail::mp_remove_impl<std::tuple<typename R0::value_type, typename Rn::value_type ...>, void>::type, boost::mp11::mp_front, typename boost::mp11::detail::mp_remove_impl<std::tuple<typename R0::value_type, typename Rn::value_type ...>, void>::type>::type> boost::urls::grammar::implementation_defined::tuple_rule_t<R0, Rn>::parse(const char*&, const char*) const [with R0 = boost::urls::detail::ipvfuture_rule_t; Rn = {boost::urls::grammar::implementation_defined::squelch_rule_t<boost::urls::grammar::implementation_defined::ch_delim_rule>}]’ at /workspace/boost/boost/url/grammar/impl/tuple_rule.hpp:291:12:
/workspace/boost/boost/url/grammar/impl/tuple_rule.hpp:223:13: error: ‘((std::size_t*)((char*)&<unnamed> + offsetof(boost::system::result<boost::urls::detail::ipvfuture_rule_t::value_type, boost::system::error_code>,boost::system::result<boost::urls::detail::ipvfuture_rule_t::value_type, boost::system::error_code>::v_.boost::variant2::variant<boost::urls::detail::ipvfuture_rule_t::value_type, boost::system::error_code>::<unnamed>.boost::variant2::detail::variant_ma_base_impl<true, true, boost::urls::detail::ipvfuture_rule_t::value_type, boost::system::error_code>::<unnamed>.boost::variant2::detail::variant_mc_base_impl<true, true, boost::urls::detail::ipvfuture_rule_t::value_type, boost::system::error_code>::<unnamed>.boost::variant2::detail::variant_ca_base_impl<true, true, boost::urls::detail::ipvfuture_rule_t::value_type, boost::system::error_code>::<unnamed>.boost::variant2::detail::variant_cc_base_impl<true, true, boost::urls::detail::ipvfuture_rule_t::value_type, boost::system::error_code>::<unnamed>.boost::variant2::detail::variant_base_impl<true, true, boost::urls::detail::ipvfuture_rule_t::value_type, boost::system::error_code>::st_)))[3]’ may be used uninitialized [-Werror=maybe-uninitialized]
  223 |             v = rv.error();
      |             ^
/workspace/boost/boost/url/grammar/impl/tuple_rule.hpp: In member function ‘constexpr boost::system::result<typename boost::mp11::detail::mp_eval_if_c_impl<boost::urls::grammar::implementation_defined::tuple_rule_t<R0, Rn>::IsList, typename boost::mp11::detail::mp_remove_impl<std::tuple<typename R0::value_type, typename Rn::value_type ...>, void>::type, boost::mp11::mp_front, typename boost::mp11::detail::mp_remove_impl<std::tuple<typename R0::value_type, typename Rn::value_type ...>, void>::type>::type> boost::urls::grammar::implementation_defined::tuple_rule_t<R0, Rn>::parse(const char*&, const char*) const [with R0 = boost::urls::detail::ipvfuture_rule_t; Rn = {boost::urls::grammar::implementation_defined::squelch_rule_t<boost::urls::grammar::implementation_defined::ch_delim_rule>}]’:
/workspace/boost/boost/url/grammar/impl/tuple_rule.hpp:223:15: note: ‘<anonymous>’ declared here
  223 |             v = rv.error();
      |             ~~^~~~~~~~~~~~
In member function ‘constexpr void boost::urls::grammar::detail::parse_sequence<false, R0, Rn ...>::apply(const char*&, const char*, boost::mp11::mp_size_t<I>&, boost::mp11::mp_size_t<Iv>&, const boost::mp11::mp_true&) [with long unsigned int Ir = 1; long unsigned int Iv = 1; R0 = boost::urls::detail::ipvfuture_rule_t; Rn = {boost::urls::grammar::implementation_defined::squelch_rule_t<boost::urls::grammar::implementation_defined::ch_delim_rule>}]’,
    inlined from ‘constexpr typename std::enable_if<(Ir < (1 + sizeof... (Rn)))>::type boost::urls::grammar::detail::parse_sequence<false, R0, Rn ...>::apply(const char*&, const char*, boost::mp11::mp_size_t<I>&, boost::mp11::mp_size_t<Iv>&) [with long unsigned int Ir = 1; long unsigned int Iv = 1; R0 = boost::urls::detail::ipvfuture_rule_t; Rn = {boost::urls::grammar::implementation_defined::squelch_rule_t<boost::urls::grammar::implementation_defined::ch_delim_rule>}]’ at /workspace/boost/boost/url/grammar/impl/tuple_rule.hpp:265:14,
    inlined from ‘constexpr void boost::urls::grammar::detail::parse_sequence<false, R0, Rn ...>::apply(const char*&, const char*, boost::mp11::mp_size_t<I>&, boost::mp11::mp_size_t<Iv>&, const boost::mp11::mp_false&) [with long unsigned int Ir = 0; long unsigned int Iv = 0; R0 = boost::urls::detail::ipvfuture_rule_t; Rn = {boost::urls::grammar::implementation_defined::squelch_rule_t<boost::urls::grammar::implementation_defined::ch_delim_rule>}]’ at /workspace/boost/boost/url/grammar/impl/tuple_rule.hpp:247:14,
    inlined from ‘constexpr typename std::enable_if<(Ir < (1 + sizeof... (Rn)))>::type boost::urls::grammar::detail::parse_sequence<false, R0, Rn ...>::apply(const char*&, const char*, boost::mp11::mp_size_t<I>&, boost::mp11::mp_size_t<Iv>&) [with long unsigned int Ir = 0; long unsigned int Iv = 0; R0 = boost::urls::detail::ipvfuture_rule_t; Rn = {boost::urls::grammar::implementation_defined::squelch_rule_t<boost::urls::grammar::implementation_defined::ch_delim_rule>}]’ at /workspace/boost/boost/url/grammar/impl/tuple_rule.hpp:265:14,
    inlined from ‘constexpr boost::system::result<typename boost::mp11::detail::mp_eval_if_c_impl<boost::urls::grammar::implementation_defined::tuple_rule_t<R0, Rn>::IsList, typename boost::mp11::detail::mp_remove_impl<std::tuple<typename R0::value_type, typename Rn::value_type ...>, void>::type, boost::mp11::mp_front, typename boost::mp11::detail::mp_remove_impl<std::tuple<typename R0::value_type, typename Rn::value_type ...>, void>::type>::type> boost::urls::grammar::implementation_defined::tuple_rule_t<R0, Rn>::parse(const char*&, const char*) const [with R0 = boost::urls::detail::ipvfuture_rule_t; Rn = {boost::urls::grammar::implementation_defined::squelch_rule_t<boost::urls::grammar::implementation_defined::ch_delim_rule>}]’ at /workspace/boost/boost/url/grammar/impl/tuple_rule.hpp:291:12:
/workspace/boost/boost/url/grammar/impl/tuple_rule.hpp:223:13: error: ‘((const char**)((char*)&<unnamed> + offsetof(boost::system::result<boost::urls::detail::ipvfuture_rule_t::value_type, boost::system::error_code>,boost::system::result<boost::urls::detail::ipvfuture_rule_t::value_type, boost::system::error_code>::v_.boost::variant2::variant<boost::urls::detail::ipvfuture_rule_t::value_type, boost::system::error_code>::<unnamed>.boost::variant2::detail::variant_ma_base_impl<true, true, boost::urls::detail::ipvfuture_rule_t::value_type, boost::system::error_code>::<unnamed>.boost::variant2::detail::variant_mc_base_impl<true, true, boost::urls::detail::ipvfuture_rule_t::value_type, boost::system::error_code>::<unnamed>.boost::variant2::detail::variant_ca_base_impl<true, true, boost::urls::detail::ipvfuture_rule_t::value_type, boost::system::error_code>::<unnamed>.boost::variant2::detail::variant_cc_base_impl<true, true, boost::urls::detail::ipvfuture_rule_t::value_type, boost::system::error_code>::<unnamed>.boost::variant2::detail::variant_base_impl<true, true, boost::urls::detail::ipvfuture_rule_t::value_type, boost::system::error_code>::st_)))[4]’ may be used uninitialized [-Werror=maybe-uninitialized]
  223 |             v = rv.error();
      |             ^
/workspace/boost/boost/url/grammar/impl/tuple_rule.hpp: In member function ‘constexpr boost::system::result<typename boost::mp11::detail::mp_eval_if_c_impl<boost::urls::grammar::implementation_defined::tuple_rule_t<R0, Rn>::IsList, typename boost::mp11::detail::mp_remove_impl<std::tuple<typename R0::value_type, typename Rn::value_type ...>, void>::type, boost::mp11::mp_front, typename boost::mp11::detail::mp_remove_impl<std::tuple<typename R0::value_type, typename Rn::value_type ...>, void>::type>::type> boost::urls::grammar::implementation_defined::tuple_rule_t<R0, Rn>::parse(const char*&, const char*) const [with R0 = boost::urls::detail::ipvfuture_rule_t; Rn = {boost::urls::grammar::implementation_defined::squelch_rule_t<boost::urls::grammar::implementation_defined::ch_delim_rule>}]’:
/workspace/boost/boost/url/grammar/impl/tuple_rule.hpp:223:15: note: ‘<anonymous>’ declared here
  223 |             v = rv.error();
      |             ~~^~~~~~~~~~~~
In member function ‘constexpr void boost::urls::grammar::detail::parse_sequence<false, R0, Rn ...>::apply(const char*&, const char*, boost::mp11::mp_size_t<I>&, boost::mp11::mp_size_t<Iv>&, const boost::mp11::mp_true&) [with long unsigned int Ir = 1; long unsigned int Iv = 1; R0 = boost::urls::detail::ipvfuture_rule_t; Rn = {boost::urls::grammar::implementation_defined::squelch_rule_t<boost::urls::grammar::implementation_defined::ch_delim_rule>}]’,
    inlined from ‘constexpr typename std::enable_if<(Ir < (1 + sizeof... (Rn)))>::type boost::urls::grammar::detail::parse_sequence<false, R0, Rn ...>::apply(const char*&, const char*, boost::mp11::mp_size_t<I>&, boost::mp11::mp_size_t<Iv>&) [with long unsigned int Ir = 1; long unsigned int Iv = 1; R0 = boost::urls::detail::ipvfuture_rule_t; Rn = {boost::urls::grammar::implementation_defined::squelch_rule_t<boost::urls::grammar::implementation_defined::ch_delim_rule>}]’ at /workspace/boost/boost/url/grammar/impl/tuple_rule.hpp:265:14,
    inlined from ‘constexpr void boost::urls::grammar::detail::parse_sequence<false, R0, Rn ...>::apply(const char*&, const char*, boost::mp11::mp_size_t<I>&, boost::mp11::mp_size_t<Iv>&, const boost::mp11::mp_false&) [with long unsigned int Ir = 0; long unsigned int Iv = 0; R0 = boost::urls::detail::ipvfuture_rule_t; Rn = {boost::urls::grammar::implementation_defined::squelch_rule_t<boost::urls::grammar::implementation_defined::ch_delim_rule>}]’ at /workspace/boost/boost/url/grammar/impl/tuple_rule.hpp:247:14,
    inlined from ‘constexpr typename std::enable_if<(Ir < (1 + sizeof... (Rn)))>::type boost::urls::grammar::detail::parse_sequence<false, R0, Rn ...>::apply(const char*&, const char*, boost::mp11::mp_size_t<I>&, boost::mp11::mp_size_t<Iv>&) [with long unsigned int Ir = 0; long unsigned int Iv = 0; R0 = boost::urls::detail::ipvfuture_rule_t; Rn = {boost::urls::grammar::implementation_defined::squelch_rule_t<boost::urls::grammar::implementation_defined::ch_delim_rule>}]’ at /workspace/boost/boost/url/grammar/impl/tuple_rule.hpp:265:14,
    inlined from ‘constexpr boost::system::result<typename boost::mp11::detail::mp_eval_if_c_impl<boost::urls::grammar::implementation_defined::tuple_rule_t<R0, Rn>::IsList, typename boost::mp11::detail::mp_remove_impl<std::tuple<typename R0::value_type, typename Rn::value_type ...>, void>::type, boost::mp11::mp_front, typename boost::mp11::detail::mp_remove_impl<std::tuple<typename R0::value_type, typename Rn::value_type ...>, void>::type>::type> boost::urls::grammar::implementation_defined::tuple_rule_t<R0, Rn>::parse(const char*&, const char*) const [with R0 = boost::urls::detail::ipvfuture_rule_t; Rn = {boost::urls::grammar::implementation_defined::squelch_rule_t<boost::urls::grammar::implementation_defined::ch_delim_rule>}]’ at /workspace/boost/boost/url/grammar/impl/tuple_rule.hpp:291:12:
/workspace/boost/boost/url/grammar/impl/tuple_rule.hpp:223:13: error: ‘((std::size_t*)((char*)&<unnamed> + offsetof(boost::system::result<boost::urls::detail::ipvfuture_rule_t::value_type, boost::system::error_code>,boost::system::result<boost::urls::detail::ipvfuture_rule_t::value_type, boost::system::error_code>::v_.boost::variant2::variant<boost::urls::detail::ipvfuture_rule_t::value_type, boost::system::error_code>::<unnamed>.boost::variant2::detail::variant_ma_base_impl<true, true, boost::urls::detail::ipvfuture_rule_t::value_type, boost::system::error_code>::<unnamed>.boost::variant2::detail::variant_mc_base_impl<true, true, boost::urls::detail::ipvfuture_rule_t::value_type, boost::system::error_code>::<unnamed>.boost::variant2::detail::variant_ca_base_impl<true, true, boost::urls::detail::ipvfuture_rule_t::value_type, boost::system::error_code>::<unnamed>.boost::variant2::detail::variant_cc_base_impl<true, true, boost::urls::detail::ipvfuture_rule_t::value_type, boost::system::error_code>::<unnamed>.boost::variant2::detail::variant_base_impl<true, true, boost::urls::detail::ipvfuture_rule_t::value_type, boost::system::error_code>::st_)))[5]’ may be used uninitialized [-Werror=maybe-uninitialized]
  223 |             v = rv.error();
      |             ^
/workspace/boost/boost/url/grammar/impl/tuple_rule.hpp: In member function ‘constexpr boost::system::result<typename boost::mp11::detail::mp_eval_if_c_impl<boost::urls::grammar::implementation_defined::tuple_rule_t<R0, Rn>::IsList, typename boost::mp11::detail::mp_remove_impl<std::tuple<typename R0::value_type, typename Rn::value_type ...>, void>::type, boost::mp11::mp_front, typename boost::mp11::detail::mp_remove_impl<std::tuple<typename R0::value_type, typename Rn::value_type ...>, void>::type>::type> boost::urls::grammar::implementation_defined::tuple_rule_t<R0, Rn>::parse(const char*&, const char*) const [with R0 = boost::urls::detail::ipvfuture_rule_t; Rn = {boost::urls::grammar::implementation_defined::squelch_rule_t<boost::urls::grammar::implementation_defined::ch_delim_rule>}]’:
/workspace/boost/boost/url/grammar/impl/tuple_rule.hpp:223:15: note: ‘<anonymous>’ declared here
  223 |             v = rv.error();
      |             ~~^~~~~~~~~~~~
cc1plus: all warnings being treated as errors

This issue occurs after the changes introduced in this commit: feat: constexpr URL parsing for C++20
.

[alandefreitas]

I have been investigating this issue for the last 12 hours, but I don't think there's anything we can (or should) do about that. This is a false positive originating in Boost.Optional, Boost.System, and Boost.Variant that wasn't being triggered for circumstantial reasons. And there's nothing Boost.Optional, Boost.System, and Boost.Variant can do to ensure this won't be triggered either, because GCC triggers it considering the call site, even if you include the libraries with -isystem or use some #pragma trick to ignore it.

I did get GCC to stop triggering the warning by adding some code to tuple_rule.hpp that wasn't functionally needed by the application, and putting force inline attributes in some functions. But this is very problematic. It didn't fix the problem in all cases covered in our tests because the error ultimately comes from other libraries. Some other tooling will identify this extra non-functional code sooner or later. All downstream users would be including, compiling, and running useless code just to avoid a warning. And there's no way to test if there are no regressions after the fix, because we'd still be ignoring the warning in our tests ourselves.

Unless there's any new information on this, I think the most reasonable solution is for any downstream users of any library to never enable warnings with false positives, or be willing to iteratively disable the ones affecting them as they arise. In fact, Boost.URL uses -Wno-unused-but-set-variable right now in its tests to avoid these very same warnings, because they come from other libraries.

[alandefreitas]

I was just talking to Peter. We'll coordinate to implement whatever changes we need in variant2. :)