Language: C++

#include "bits/stdc++.h"

// Begin Header {{{
using namespace std;

#ifndef DEBUG
#define dump(...)
#endif

#define all(x) x.begin(), x.end()
#define rep(i, n) for (intmax_t i = 0, i##_limit = (n); i < i##_limit; ++i)
#define reps(i, b, e) for (intmax_t i = (b), i##_limit = (e); i <= i##_limit; ++i)
#define repr(i, b, e) for (intmax_t i = (b), i##_limit = (e); i >= i##_limit; --i)
#define var(Type, ...) Type __VA_ARGS__; input(__VA_ARGS__)

constexpr size_t    operator""_zu(unsigned long long value) { return value; };
constexpr intmax_t  operator""_jd(unsigned long long value) { return value; };
constexpr uintmax_t operator""_ju(unsigned long long value) { return value; };

constexpr int INF = 0x3f3f3f3f;
constexpr intmax_t LINF = 0x3f3f3f3f3f3f3f3f_jd;

template <class T>
using MaxHeap = priority_queue<T, vector<T>, less<T>>;
template <class T>
using MinHeap = priority_queue<T, vector<T>, greater<T>>;

inline void input() {}
template <class Head, class... Tail>
inline void input(Head&& head, Tail&&... tail) {
    cin >> head;
    input(forward<Tail>(tail)...);
}

template <class T>
inline void input(vector<T> &vec) {
    for (auto &e: vec) {
        cin >> e;
    }
}

template <class T>
inline void input(vector<vector<T>> &mat) {
    for (auto &vec: mat) {
        input(vec);
    }
}

inline void print() { cout << "\n"; }
template <class Head, class... Tail>
inline void print(Head&& head, Tail&&... tail) {
    cout << head;
    if (sizeof...(tail)) cout << " ";
    print(forward<Tail>(tail)...);
}

template <class T>
inline ostream& operator<<(ostream &strm, const vector<T> &vec) {
    static constexpr const char *delim[] = {" ", ""};
    for (const auto &e: vec) {
        strm << e << delim[&e == &vec.back()];
    }
    return strm;
}

template <class T>
inline vector<T> make_v(const T &initValue, size_t sz) {
    return vector<T>(sz, initValue);
}

template <class T, class... Args>
inline auto make_v(const T &initValue, size_t sz, Args... args) {
    return vector<decltype(make_v<T>(initValue, args...))>(sz, make_v<T>(initValue, args...));
}

template <class Func>
class FixPoint : Func {
public:
    explicit constexpr FixPoint(Func&& f) noexcept : Func(forward<Func>(f)) {}

    template <class... Args>
    constexpr decltype(auto) operator()(Args&&... args) const {
        return Func::operator()(*this, std::forward<Args>(args)...);
    }
};

template <class Func>
static inline constexpr decltype(auto) makeFixPoint(Func&& f) noexcept {
    return FixPoint<Func>{forward<Func>(f)};
}

template <class T>
inline bool chmax(T &a, const T &b) noexcept {
    return b > a && (a = b, true);
}

template <class T>
inline bool chmin(T &a, const T &b) noexcept {
    return b < a && (a = b, true);
}

template <class T>
inline T diff(const T &a, const T &b) noexcept {
    return a < b ? b - a : a - b;
}
// End Header }}}

signed main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    var(size_t, N, M);

    vector<size_t> num(N + 1, 1);
    vector<bool> exist(N + 1, false);
    exist[1] = true;
    rep(i, M) {
        var(size_t, x, y);

        num[x]--;
        num[y]++;
        exist[y] |= exist[x];

        if (num[x] == 0) {
            exist[x] = false;
        }
    }

    print(count(all(exist), true));

    return 0;
}


Compiler output:
a.cc: In function 'int main()':
a.cc:122:18: error: no match for 'operator|=' (operand types are 'std::vector<bool>::reference' and 'std::vector<bool>::reference')
  122 |         exist[y] |= exist[x];
      |         ~~~~~~~~~^~~~~~~~~~~
In file included from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:41,
                 from a.cc:1:
/usr/include/c++/14/cstddef:168:3: note: candidate: 'constexpr std::byte& std::operator|=(byte&, byte)'
  168 |   operator|=(byte& __l, byte __r) noexcept
      |   ^~~~~~~~
/usr/include/c++/14/cstddef:168:20: note:   no known conversion for argument 1 from 'std::vector<bool>::reference' to 'std::byte&'
  168 |   operator|=(byte& __l, byte __r) noexcept
      |              ~~~~~~^~~
In file included from /usr/include/c++/14/streambuf:43,
                 from /usr/include/c++/14/bits/streambuf_iterator.h:35,
                 from /usr/include/c++/14/iterator:66,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:54:
/usr/include/c++/14/bits/ios_base.h:104:3: note: candidate: 'constexpr const std::_Ios_Fmtflags& std::operator|=(_Ios_Fmtflags&, _Ios_Fmtflags)'
  104 |   operator|=(_Ios_Fmtflags& __a, _Ios_Fmtflags __b) _GLIBCXX_NOTHROW
      |   ^~~~~~~~
/usr/include/c++/14/bits/ios_base.h:104:29: note:   no known conversion for argument 1 from 'std::vector<bool>::reference' to 'std::_Ios_Fmtflags&'
  104 |   operator|=(_Ios_Fmtflags& __a, _Ios_Fmtflags __b) _GLIBCXX_NOTHROW
      |              ~~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/ios_base.h:154:3: note: candidate: 'constexpr const std::_Ios_Openmode& std::operator|=(_Ios_Openmode&, _Ios_Openmode)'
  154 |   operator|=(_Ios_Openmode& __a, _Ios_Openmode __b) _GLIBCXX_NOTHROW
      |   ^~~~~~~~
/usr/include/c++/14/bits/ios_base.h:154:29: note:   no known conversion for argument 1 from 'std::vector<bool>::reference' to 'std::_Ios_Openmode&'
  154 |   operator|=(_Ios_Openmode& __a, _Ios_Openmode __b) _GLIBCXX_NOTHROW
      |              ~~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/ios_base.h:201:3: note: candidate: 'constexpr const std::_Ios_Iostate& std::operator|=(_Ios_Iostate&, _Ios_Iostate)'
  201 |   operator|=(_Ios_Iostate& __a, _Ios_Iostate __b) _GLIBCXX_NOTHROW
      |   ^~~~~~~~
/usr/include/c++/14/bits/ios_base.h:201:28: note:   no known conversion for argument 1 from 'std::vector<bool>::reference' to 'std::_Ios_Iostate&'
  201 |   operator|=(_Ios_Iostate& __a, _Ios_Iostate __b) _GLIBCXX_NOTHROW
      |              ~~~~~~~~~~~~~~^~~
In file included from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:176:
/usr/include/c++/14/future:185:18: note: candidate: 'constexpr std::launch& std::operator|=(launch&, launch)'
  185 |   inline launch& operator|=(launch& __x, launch __y) noexcept
      |                  ^~~~~~~~
/usr/include/c++/14/future:185:37: note:   no known conversion for argument 1 from 'std::vector<bool>::reference' to 'std::launch&'
  185 |   inline launch& operator|=(launch& __x, launch __y) noexcept
      |                             ~~~~~~~~^~~
In file included from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:198:
/usr/include/c++/14/charconv:646:3: note: candidate: 'constexpr std::chars_format& std::operator|=(chars_format&, chars_format)'
  646 |   operator|=(chars_format& __lhs, chars_format __rhs) noexcept
      |   ^~~~~~~~
/usr/include/c++/14/charconv:646:28: note:   no known conversion for argument 1 from 'std::vector<bool>::reference' to 'std::chars_format&'
  646 |   operator|=(chars_format& __lhs, chars_format __rhs) noexcept
      |              ~~~~~~~~~~~~~~^~~~~