trailing commas and comments not working well in some cases

[terrywh]

Version of Boost

1.79.0

Steps necessary to reproduce the problem

#include <boost/json/parse.hpp>
#include <boost/json/value.hpp>

int main(int argc, char* argv[]) {
    std::string raw = R"([
    "123", // 456
])";
    boost::json::parse_options opt {};
    opt.allow_comments = true;
    opt.allow_trailing_commas = true;
    boost::json::value val = boost::json::parse(raw, {}, opt);
    return 0;
}

compile :

g++ -I /data/vendor/boost-1.79/include -L /data/vendor/boost-1.79/lib test.cpp -lboost_json -lboost_system -o test_json

run the program, and it will terminate with the following:

terminate called after throwing an instance of 'boost::wrapexcept<boost::system::system_error>'
  what():  syntax error [boost.json:1 at ./boost/json/basic_parser_impl.hpp:634 in function 'parse_value']

either remove the comment ("// 456") or the trailing commas, the exception will go away.

All relevant compiler information

gcc/g++ 7.3.1