X3: Vector of variant breaks string attributes if variant also contains character type

[sehe]

Observed from here, reproducer:

Live On Coliru:

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

namespace x3 = boost::spirit::x3;
constexpr static inline std::string_view sample = R"("asdf" "xxx" "yyy")";

template <typename CharType> size_t test() {
    static auto const qstr                      //
        = x3::rule<class _, std::string>{"str"} //
        = x3::lexeme['"' >> *~x3::char_('"') >> '"'];

    using Var = boost::variant<std::string, CharType>;

    std::vector<Var> data;
    phrase_parse(sample.begin(), sample.end(), *qstr, x3::space, data);
    return data.size();
}

#define TEST(CharType) \
    std::cout << std::setw(13) << #CharType << " -> " << test<CharType>() << std::endl;

int main() {
    TEST(uint8_t);
    TEST(int8_t);
    TEST(signed char);
    TEST(unsigned char);

    TEST(char);
}

Prints

      uint8_t -> 3
       int8_t -> 3
  signed char -> 3
unsigned char -> 3
         char -> 10

As we can see, having char inside the variant breaks string parsing, instead treating the vector as the "string container", filling it with the individual characters.

This breaking change happened in c60d93f which was "bug fix for x3 where container attribute is substitute for the container value type" (indeed the output was consistent with boost 1.58.0)

I think the exception introduced there should not apply here as the rule declares an attribute of std::string explicitly, and as such should not expect vector<variant<..., char>> to be a match.

[saki7]

This could be either an intended behavior or a misfeature. I'm not sure which is correct. I'm also not sure if this issue persists on current codebase (as of 2025.)