Sequence parser overwrites std::u32string

[slavaandrejev]

Consider the following example.

#include <print>
#include <string>

#include <boost/parser/parser.hpp>

namespace bp = boost::parser;

int main(int argc, char *argv[]) {
    auto input = std::string{"ab"};
    auto res   = std::string();
    const auto result = bp::parse(
        input
      , (bp::char_ >> bp::char_)
      , res
      );
    if (result) {
        std::print("Parse successful\n");
        std::print("{}\n", res);
    } else {
        std::print("Parse failed\n");
    }
}

It outputs

Parse successful
ab

Now consider this change

#include <print>
#include <ranges>
#include <string>

#include <boost/parser/parser.hpp>
#include <boost/parser/transcode_view.hpp>

namespace bp = boost::parser;
namespace rs = std::ranges;

int main(int argc, char *argv[]) {
    auto input = std::u32string{U"ab"};
    auto res   = std::u32string();
    const auto result = bp::parse(
        input
      , (bp::char_ >> bp::char_)
      , res
      );
    if (result) {
        std::print("Parse successful\n");
        std::print("{}\n", res | bp::as_utf8 | rs::to<std::string>());
    } else {
        std::print("Parse failed\n");
    }
}

It outputs

Parse successful
b

The issue is here:

                    if constexpr (detail::is_nope_v<attr_t>) {
                        // nothing to do
                    } if constexpr (
                        (!out_container ||
                         !std::is_same_v<just_x, just_out>) &&
                        std::is_assignable_v<just_out &, just_x &&> &&
                        (!std::is_same_v<just_out, std::string> ||
                         !std::is_integral_v<just_x>)) {
                        detail::assign(out, std::move(x));
                    } else {
                        detail::move_back(
                            out, std::move(x), detail::gen_attrs(flags));
                    }

This code adds to a string only if it is an std::string. Otherwise, it assigns every character to the same string.

[elfring]

I became also curious how the affected software component will evolve further.

parser/include/boost/parser/parser.hpp

Lines 4583 to 4595 in 85320bd

	if constexpr (detail::is_nope_v<attr_t>) {
	// nothing to do
	} else if constexpr (
	(!out_container ||
	!std::is_same_v<just_x, just_out>) &&
	std::is_assignable_v<just_out &, just_x &&> &&
	(!std::is_same_v<just_out, std::string> ||
	!std::is_integral_v<just_x>)) {
	detail::assign(out, std::move(x));
	} else {
	detail::move_back(
	out, std::move(x), detail::gen_attrs(flags));
	}