Synthesized attributes of partial matches from alternatives - c++

Question

When I remove x3::eps in the below rule, the string result from the first partial match is still in the second match, resulting in a string with duplicated content.
If I add another case in between I still only get 1 duplicate instead of two.
Why do I need to use x3::eps, or, what am I misunderstanding in the evaluation of rules and synthesized attributes?
Should I use lookahead instead?
#include <boost/config/warning_disable.hpp>
#include <boost/spirit/home/x3.hpp>
#include <boost/spirit/include/qi_char_class.hpp>
#include <boost/fusion/include/adapt_struct.hpp>
#include <iostream>
#include <string>
#include <vector>
namespace x3 = boost::spirit::x3;
namespace ascii = x3::ascii;
struct AstChannel {
std::string label;
bool complement;
};
x3::rule<class AstLabel, std::string> const astLabel = "astLabel";
auto const astLabel_def = ascii::lower >> *(ascii::alnum);
BOOST_SPIRIT_DEFINE(astLabel)
x3::rule<class AstChannel, AstChannel> const astChannel = "astChannel";
auto const astChannel_def = astLabel >> '!' >> x3::attr(true)
| astLabel >> x3::eps >> x3::attr(false) ;
BOOST_SPIRIT_DEFINE(astChannel)
BOOST_FUSION_ADAPT_STRUCT(
AstChannel,
(std::string, label)
(bool, complement)
)
int main() {
std::string str("hello");
auto iter = str.begin();
auto end = str.end();
AstChannel channel;
bool r = phrase_parse(iter, end, astChannel, ascii::space, channel);
if (r) {
std::cout << channel.label << ',' << channel.complement << std::endl;
}
return 0;
}

Answer 1

The real answer is: force atomic attribute propagation of container attributes (e.g. with x3::hold or semantic actions).
The better answer is: use x3::matches:
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#include <boost/spirit/home/x3.hpp>
#include <boost/fusion/include/adapt_struct.hpp>
#include <iostream>
using namespace std::string_literals;
struct AstChannel {
std::string label;
bool complement;
};
BOOST_FUSION_ADAPT_STRUCT(AstChannel, label, complement)
namespace Grammar {
namespace x3 = boost::spirit::x3;
namespace ascii = x3::ascii;
auto const label = x3::rule<struct labelRule, std::string> {"label" }
= x3::lexeme[ascii::lower >> *(ascii::alnum)];
auto const channel = label >> x3::matches['!'];
auto const entry = x3::skip(ascii::space) [ channel ];
}
int main() {
auto const str = "hello"s;
AstChannel channel;
if (parse(str.begin(), str.end(), Grammar::entry, channel)) {
std::cout << channel.label << ',' << std::boolalpha << channel.complement << "\n";
}
}
Prints
hello,false