Language: C++

#include <iostream>
#include <iomanip>
#include <vector>
#include <valarray>
#include <map>
#include <set>
#include <list>
#include <queue>
#include <stack>
#include <bitset>
#include <utility>
#include <numeric>
#include <algorithm>
#include <functional>
#include <complex>
#include <string>
#include <sstream>
#include <array>

#include <cstdio>
#include <cstdlib>
#include <cctype>
#include <cstring>
#include <cassert>

#include <unordered_set>
#include <unordered_map>
#include <random>
#include <thread>
#include <chrono>

using namespace std;

#define all(c) c.begin(), c.end()
#define repeat(i, n) for (int i = 0; i < static_cast<int>(n); i++)
#define debug(x) #x << "=" << (x)

#ifdef DEBUG
#define dump(x) std::cerr << debug(x) << " (L:" << __LINE__ << ")" << std::endl
#else
#define dump(x)
#endif


template <typename T>
ostream &operator<<(ostream &os, const vector<T> &v)
{
    os << "{";
    for(auto it = v.begin(); it != v.end(); ++it) {
        if(it != v.begin())
            os << ",";
        os << *it;
    }
    return os << "}";
}

template <typename T>
ostream& operator<<(ostream& os, const set<T>& v)
{
    os << "{";
    for (auto it = v.begin(); it != v.end(); ++it) {
        if (it != v.begin())
            os << ",";
        os << *it;
    }
    return os << "}";
}

template <typename A, typename B>
ostream &operator<<(ostream &os, const pair<A, B> &v)
{
    os << "{";
    os << v.first << ", " << v.second;
    return os << "}";
}

// ---------------------------------------------------------------------------




using point = complex<double>;
using ll = int64_t;

void check(int m, int k)
{
    vector<int> arr(pow(2, m + 1));

    for (int i = 0; i < arr.size(); i++) {
        arr[i] = i / 2;
    }


    do {
        set<int> xors;
        vector<vector<int>> indexes(pow(2, m));

        for (int i = 0; i < arr.size(); i++) {
            indexes[arr[i]].push_back(i);
        }

        for (int i = 0; i < indexes.size(); i++) {
            int x = 0;

            int a = indexes[i][0];
            int b = indexes[i][1];
            for (int k = a; k <= b; k++) {
                x ^= arr[k];
            }
            xors.insert(x);
            if (xors.size() == 2)
                break;
        }

        if (xors.size() == 1 && *xors.begin() != 0) {
            cout << arr << " -> " << xors << endl;
            //dump(arr);
            //dump(xors);
        }
    } while (next_permutation(all(arr)));
}


//vector<int> check2(int m, int k)
//{
//    vector<int> arr(pow(2, m + 1));
//
//    for (int i = 0; i < arr.size(); i++) {
//        arr[i] = i / 2;
//    }
//
//    do {
//
//        set<int> xors;
//        vector<vector<int>> indexes(pow(2, m));
//        for (int i = 0; i < arr.size(); i++) {
//            indexes[arr[i]].push_back(i);
//        }
//
//        for (int i = 0; i < indexes.size(); i++) {
//
//            int a = indexes[i][0];
//            int b = indexes[i][1];
//            int x = 0;
//
//            for (int k = a; k <= b; k++) {
//                x ^= arr[k];
//            }
//            xors.insert(x);
//            if (xors.size() == 2)
//                break;
//        }
//
//        if (xors.size() == 1 && *xors.begin() == k) {
//            return arr;
//        }
//    } while (next_permutation(all(arr)));
//
//    return vector<int>();
//}

int my_main(int argc, char** argv)
{
    ios::sync_with_stdio(false);
    cin.tie(0);

    int m, k;
    cin >> m >> k;

    //check(m, k);

    int m2 = pow(2, m + 1);
    if (k == 0) {
        for (int i = 0; i < m2; i++) {
            cout << i / 2 << " ";
        }
        cout << endl;
        return 0;
    }

    if (k < m2) {
        vector<int> ret;
        ret.push_back(0);
        ret.push_back(k);
        ret.push_back(0);

        for (int i = 1; i < m2; i++) {
            if (i == k)
                continue;
            ret.push_back(i);
        }
        for (int i = m2 - 1; i >= 1; i--) {
            if (i == k)
                continue;
            ret.push_back(i);
        }

        for (int i : ret) {
            cout << i << " ";
        }
        cout << endl;
        return;
    }

    cout << -1 << endl;
    return 0;
}





// ----------------------------------------------------------------------------
// Test driver

#ifdef DEBUG
#define MAY_RUN_TESTCASE
#endif

#ifdef MAY_RUN_TESTCASE
#include <fstream>

#if __has_include(<unistd.h>)
// Mac, Linux
#include <unistd.h>
#elif __has_include(<io.h>)
// Windows
#include <io.h>
#else
// Other?
#error "unistd.h or io.h not found"
#endif

bool test_file_exists(const std::string& str)
{
    std::ifstream fs(str);
    return fs.is_open();
}

#endif

int main(int argc, char** argv)
{
#ifndef MAY_RUN_TESTCASE
    return my_main(argc, argv);
#else
    if (argc == 1) {
        return my_main(argc, argv);
    }
    else if (argc == 2) {

        char* stdin_file = argv[1];
        freopen(stdin_file, "r", stdin);
        return my_main(argc, argv);

    } else if (argc == 5) {

        std::string stdin_file = argv[1];
        std::string expected_file = argv[2];
        std::string stdout_file = argv[3];
        std::string stderr_file = argv[4];

        if (!test_file_exists(stdin_file)) {
            std::cerr << stdin_file << " not found" << std::endl;
            return 3;
        }

        if (!test_file_exists(expected_file)) {
            std::cerr << expected_file << " not found" << std::endl;
            return 3;
        }

        int original_stdin = dup(fileno(stdin));
        int original_stdout = dup(fileno(stdout));
        int original_stderr = dup(fileno(stderr));

        freopen(stdin_file.c_str(), "r", stdin);
        freopen(stdout_file.c_str(), "w", stdout);
        freopen(stderr_file.c_str(), "w", stderr);

        int ret = my_main(argc, argv);

        fflush(stdout);
        fflush(stderr);

        dup2(original_stderr, fileno(stderr));
        dup2(original_stdout, fileno(stdout));
        dup2(original_stdin, fileno(stdin));

        if (ret != 0) {
            std::cerr << "main returns " << ret << std::endl;
            return ret;
        }

        std::ifstream inp(stdin_file);
        std::ifstream out(stdout_file);
        std::ifstream err(stderr_file);
        std::ifstream exp(expected_file);

        // Clear is needed if the file is empty.
        std::cout << "----- input -----" << std::endl;
        std::cout << inp.rdbuf() << std::endl;
        std::cout.clear();
        std::cout << "-----------------" << std::endl << std::endl;

        std::cout << "----- output ----" << std::endl;
        std::cout << out.rdbuf() << std::endl;
        std::cout.clear();
        std::cout << "-----------------" << std::endl << std::endl;

        std::cout << "---- expected ---" << std::endl;
        std::cout << exp.rdbuf() << std::endl;
        std::cout.clear();
        std::cout << "-----------------" << std::endl << std::endl;

        std::cout << "----- stderr ----" << std::endl;
        std::cout << err.rdbuf() << std::endl;
        std::cout.clear();
        std::cout << "-----------------" << std::endl;

        inp.seekg(0);
        out.seekg(0);
        exp.seekg(0);
        err.seekg(0);

        std::string output_str, expected_str;
        {
            std::stringstream output_ss;
            output_ss << out.rdbuf();
            output_str = output_ss.str();

            std::stringstream expected_ss;
            expected_ss << exp.rdbuf();
            expected_str = expected_ss.str();
        }

        // Remove trailing spaces
        output_str.erase(output_str.find_last_not_of(" \n\r\t") + 1);
        expected_str.erase(expected_str.find_last_not_of(" \n\r\t") + 1);

        if (output_str == expected_str)
            return 0;
        else
            return 1;
    }
    return 1;
#endif
}


Compiler output:
a.cc: In function 'int my_main(int, char**)':
a.cc:202:9: error: return-statement with no value, in function returning 'int' [-fpermissive]
  202 |         return;
      |         ^~~~~~