boost::regex_search contradicts std::regex_search in changing match_result argument

[Artalus]

#include <iostream>
#include <regex>
#include <boost/regex.hpp>
#include <string>
  
std::string target("821-xxxzzz");
void st() {
    std::smatch sm;
    std::regex re1("^.+\\/(\\d+)$");
    std::regex_search(target, sm, re1);
    std::cout << "after std search, smatch is " << sm.size() << std::endl;
}
void bst() {
    boost::smatch sm;
    boost::regex re1("^.+\\/(\\d+)$");
    boost::regex_search(target, sm, re1);
    std::cout << "after boost search, smatch is " << sm.size() << std::endl;
}

int main() {
    st();
    bst();
    return 0;
}

after std search, smatch is 0
after boost search, smatch is 2

STD (cppreference.com) says:
the object match is updated as follows: If the match does not exist: match.ready() == true; match.empty() == true; match.size() == 0
While Boost docs says:
Postconditions: If the function returns false, then the effect on parameter match is undefined

This assumption contradicts with the logic and expectations of end-user, that if no match was found in text, then the boost::match_result object state would represent it through empty and size methods. Everything said is related to boost::regex_match too.

[degski]

@Artalus I was going to say to file an issue at boost::regex, but then I saw you already did. I'm just writing this so others know you files the issue as well. You could just close it here.