Interesting (possibly wrong) synthesized attribute behavior

gistfile1.txt

#include <iostream>
#include <boost/spirit/include/qi.hpp>
#include <boost/optional.hpp>
#include <boost/optional/optional_io.hpp>

int main() {
    using namespace std;

    // parsing parentheses that may or may not contain integers
    string in("(10) () (20)");
    vector<boost::optional<int>> result;
    auto beg = in.begin();
    auto end = in.end();
    using namespace boost::spirit::qi;
    phrase_parse(beg, end,   
                 *('(' > ((int_|eps) ^ eps) > ')'),   // <<<< second in sequence uninit instead of "not present"
                 ascii::space,
                 result);
    copy(result.begin(), result.end(),
         ostream_iterator<boost::optional<int>>(cout, " "));
    cout << "\n";
}