Compilation error in 1.77

[syyyr]

Hi, since Boost 1.77, I am not able to compile my project that uses Spirit X3. I read in the changelog, that there were some breaking changes, but I'm not sure if that's what's affecting me. To be honest, I am kinda lost in the error message. I am not sure what it means. This is the compilation error https://gist.github.com/syyyr/3ee4bac793a0724b4976138ec35f42ed. My grammars can be seen here https://github.com/CESNET/netconf-cli/blob/master/src/grammars.hpp, the AST is in the other files next to this one, but I think that https://github.com/CESNET/netconf-cli/blob/master/src/ast_path.hpp is where the problem might be. Could you please help me solve this issue?

[djowel]

I haven't investigated, but have you tried incrementally minimizing the grammar/ast to see the problem spot? It should be possible to narrow it down to something minimal.

[Kojoley]

/opt/br2-external-build/per-package/netconf-cli/host/arm-buildroot-linux-gnueabihf/sysroot/usr/include/boost/spirit/home/x3/support/traits/move_to.hpp: In instantiation of ‘void boost::spirit::x3::traits::move_to(Source&&, Dest&) [with Source = boost::variant<schemaPath_, dataPath_>; Dest = dataPath_]’:
/opt/br2-external-build/per-package/netconf-cli/host/arm-buildroot-linux-gnueabihf/sysroot/usr/include/boost/spirit/home/x3/nonterminal/detail/transform_attribute.hpp:30:28:   required from ‘static void boost::spirit::x3::default_transform_attribute<Exposed, Transformed>::post(Exposed&, Transformed&&) [with Exposed = dataPath_; Transformed = boost::variant<schemaPath_, dataPath_>]’
/opt/br2-external-build/per-package/netconf-cli/host/arm-buildroot-linux-gnueabihf/sysroot/usr/include/boost/spirit/home/x3/nonterminal/detail/rule.hpp:342:32:   required from ‘static bool boost::spirit::x3::detail::rule_parser<Attribute, ID, skip_definition_injection>::call_rule_definition(const RHS&, const char*, Iterator&, const Iterator&, const Context&, ActualAttribute&, ExplicitAttrPropagation) [with RHS = PathParser<PathParserMode::AnyPath, CompletionMode::Schema>; Iterator = __gnu_cxx::__normal_iterator<const char*, std::__cxx11::basic_string<char> >; Context = boost::spirit::x3::context<writableOps_tag, const WritableOps, boost::spirit::x3::context<boost::spirit::x3::error_handler_tag, std::reference_wrapper<boost::spirit::x3::error_handler<__gnu_cxx::__normal_iterator<const char*, std::__cxx11::basic_string<char> > > >, boost::spirit::x3::context<parser_context_tag, ParserContext, boost::spirit::x3::context<boost::spirit::x3::skipper_tag, const boost::spirit::x3::char_class<boost::spirit::char_encoding::standard, boost::spirit::x3::space_tag>, boost::spirit::x3::unused_type> > > >; ActualAttribute = dataPath_; ExplicitAttrPropagation = mpl_::bool_<false>; Attribute = boost::variant<schemaPath_, dataPath_>; ID = anyPath_class; bool skip_definition_injection = true]’
/opt/br2-external-build/per-package/netconf-cli/host/arm-buildroot-linux-gnueabihf/sysroot/usr/include/boost/spirit/home/x3/nonterminal/rule.hpp:62:39:   required from ‘bool boost::spirit::x3::rule_definition<ID, RHS, Attribute, force_attribute_, skip_definition_injection>::parse(Iterator&, const Iterator&, const Context&, boost::spirit::x3::unused_type, Attribute_&) const [with Iterator = __gnu_cxx::__normal_iterator<const char*, std::__cxx11::basic_string<char> >; Context = boost::spirit::x3::context<writableOps_tag, const WritableOps, boost::spirit::x3::context<boost::spirit::x3::error_handler_tag, std::reference_wrapper<boost::spirit::x3::error_handler<__gnu_cxx::__normal_iterator<const char*, std::__cxx11::basic_string<char> > > >, boost::spirit::x3::context<parser_context_tag, ParserContext, boost::spirit::x3::context<boost::spirit::x3::skipper_tag, const boost::spirit::x3::char_class<boost::spirit::char_encoding::standard, boost::spirit::x3::space_tag>, boost::spirit::x3::unused_type> > > >; Attribute_ = dataPath_; ID = anyPath_class; RHS = PathParser<PathParserMode::AnyPath, CompletionMode::Schema>; Attribute = boost::variant<schemaPath_, dataPath_>; bool force_attribute_ = false; bool skip_definition_injection = true]’

Rule with ID anyPath_class requested conversion from boost::variant<schemaPath_, dataPath_> to dataPath_, which is unsafe and not provided.

/opt/br2-external-build/per-package/netconf-cli/host/arm-buildroot-linux-gnueabihf/sysroot/usr/include/boost/spirit/home/x3/operator/alternative.hpp:41:45:   required from ‘bool boost::spirit::x3::alternative<Left, Right>::parse(Iterator&, const Iterator&, const Context&, RContext&, Attribute&) const [with Iterator = __gnu_cxx::__normal_iterator<const char*, std::__cxx11::basic_string<char> >; Context = boost::spirit::x3::context<writableOps_tag, const WritableOps, boost::spirit::x3::context<boost::spirit::x3::error_handler_tag, std::reference_wrapper<boost::spirit::x3::error_handler<__gnu_cxx::__normal_iterator<const char*, std::__cxx11::basic_string<char> > > >, boost::spirit::x3::context<parser_context_tag, ParserContext, boost::spirit::x3::context<boost::spirit::x3::skipper_tag, const boost::spirit::x3::char_class<boost::spirit::char_encoding::standard, boost::spirit::x3::space_tag>, boost::spirit::x3::unused_type> > > >; RContext = ls_; Attribute = boost::variant<dataPath_, schemaPath_, module_>; Left = boost::spirit::x3::rule_definition<anyPath_class, PathParser<PathParserMode::AnyPath, CompletionMode::Schema>, boost::variant<schemaPath_, dataPath_>, false, true>; Right = boost::spirit::x3::sequence<boost::spirit::x3::rule<module_class, module_>, boost::spirit::x3::literal_char<boost::spirit::char_encoding::standard, boost::spirit::x3::unused_type> >]’
/opt/br2-external-build/per-package/netconf-cli/host/arm-buildroot-linux-gnueabihf/sysroot/usr/include/boost/spirit/home/x3/operator/detail/sequence.hpp:253:34:   [ skipping 40 instantiation contexts, use -ftemplate-backtrace-limit=0 to disable ]

I cannot for sure say where that happens (path is truncated by GCC [ skipping 40 instantiation contexts, use -ftemplate-backtrace-limit=0 to disable ]), but rule<anyPath_class, ...> | rule<module_class, ...> >> lit(...) most likely is here https://github.com/CESNET/netconf-cli/blob/2e092b324796bd869e3f5375c09656b0897786ae/src/path_parser.hpp#L341-L343, the attribute of the parser is struct ls_ https://github.com/CESNET/netconf-cli/blob/2e092b324796bd869e3f5375c09656b0897786ae/src/ast_commands.hpp#L38-L58

IIUC the idea is that:

• *(space_separator >> ls_options) goes into std::vector<LsOption> m_options;, and
• -(space_separator >> (anyPath | (module >> "*"))) goes into boost::optional<boost::variant<dataPath_, schemaPath_, module_>> m_path;.

where

• anyPath produces AnyPath which is using AnyPath = boost::variant<schemaPath_, dataPath_>;, and
• module produces module_

so anyPath | (module >> "*") (boost::variant<dataPath_, schemaPath_> | module_) parses into boost::variant<dataPath_, schemaPath_, module_>, which looks fine and should work.

I suspect https://github.com/boostorg/spirit/pull/665 uncovered some bug, @syyyr could you try reverting it locally?

[Kojoley]

Yep, there is a long standing bug in the alternative parser

#include <boost/spirit/home/x3.hpp>
#include <boost/variant.hpp>
#include <iostream>

namespace x3 = boost::spirit::x3;

struct X {};
struct Y {};
struct Z {};

int main()
{
    boost::variant<X, Y, Z> v;
    boost::variant<Y, Z> a;
    v = a; // variant supports that convertion
    char const* s = "";
    parse(s, s, x3::attr(boost::variant<Y, X>{}) | x3::attr(Z{}), v);
}

https://godbolt.org/z/T6vM3r6cr

[Kojoley]

@syyyr PathParser https://github.com/CESNET/netconf-cli/blob/2e092b324796bd869e3f5375c09656b0897786ae/src/path_parser.hpp#L267-L268 must have static bool const has_attribute = true;. After this fix you could replace https://github.com/CESNET/netconf-cli/blob/2e092b324796bd869e3f5375c09656b0897786ae/src/path_parser.hpp#L341

auto const anyPath = x3::rule<class anyPath_class, AnyPath>{"anyPath"} = PathParser<PathParserMode::AnyPath, CompletionMode::Schema>{};

with

const PathParser<PathParserMode::AnyPath, CompletionMode::Schema> anyPath{};

and that hopefully will workaround the issue.

[Kojoley]

Also https://github.com/CESNET/netconf-cli/blob/2e092b324796bd869e3f5375c09656b0897786ae/src/path_parser.hpp#L268 should have been using attribute_type = typename ModeToAttribute<PARSER_MODE>::type;

[syyyr]

Hi, thank you for looking into this.

The problem does appear after #665. I tried applying your suggested changes, but I can't get it to compile. This is what I did:

diff --git a/src/path_parser.hpp b/src/path_parser.hpp
index 0aace94..f60789c 100644
--- a/src/path_parser.hpp
+++ b/src/path_parser.hpp
@@ -265,7 +265,8 @@ struct ModeToAttribute<PathParserMode::DataPathListEnd> {

 template <PathParserMode PARSER_MODE, CompletionMode COMPLETION_MODE>
 struct PathParser : x3::parser<PathParser<PARSER_MODE, COMPLETION_MODE>> {
-    using attribute_type = ModeToAttribute<PARSER_MODE>;
+    using attribute_type = typename ModeToAttribute<PARSER_MODE>::type;
+    static bool const has_attribute = true;
     std::function<bool(const Schema&, const std::string& path)> m_filterFunction;

     PathParser(const std::function<bool(const Schema&, const std::string& path)>& filterFunction = [](const auto&, const auto&) { return true; })
@@ -338,7 +339,7 @@ struct PathParser : x3::parser<PathParser<PARSER_MODE, COMPLETION_MODE>> {
 // The PathParser class would get a boost::variant as the attribute, but I
 // don't want to deal with that, so I use these wrappers to ensure the
 // attribute I want (and let Spirit deal with boost::variant).
-auto const anyPath = x3::rule<class anyPath_class, AnyPath>{"anyPath"} = PathParser<PathParserMode::AnyPath, CompletionMode::Schema>{};
+const PathParser<PathParserMode::AnyPath, CompletionMode::Schema> anyPath{};
 auto const dataPath = x3::rule<class dataPath_class, dataPath_>{"dataPath"} = PathParser<PathParserMode::DataPath, CompletionMode::Data>{};
 auto const dataPathListEnd = x3::rule<class dataPath_class, dataPath_>{"dataPath"} = PathParser<PathParserMode::DataPathListEnd, CompletionMode::Data>{};

The original error is fixed, but I get another error: https://gist.github.com/syyyr/edeb2bdd66ae345e6843d3fc8ea04a59 I think the problem now is that Spirit is giving the wrong type to attr. I usually fix that problem by wrapping my parsers with x3::rule and enforce the correct attribute, but you suggested that I get rid of the x3::rule. Maybe something else is missing?

[Kojoley]

Unfortunately, if your parser produces a variant type and you use it in an alternative parser which parses into a variant attribute which is not exactly the same type (e.g. "broader") -- you will hit the bug. If you cannot skip the release you could essentially get a fix https://github.com/boostorg/spirit/pull/702 by overriding the variant_find_substitute with a specialization:

#if BOOST_VERSION <= 107700
namespace boost { namespace spirit { namespace x3 { namespace traits
{
    // backport the fix https://github.com/boostorg/spirit/pull/702
    template <typename... Types, typename T>
    struct variant_find_substitute<boost::variant<Types...>, T>
    {
       using variant_type = boost::variant<Types...>;
       ... // copy the rest content of the fixed variant_find_substitute
    }
}
#endif