Language: C++

#pragma once

#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <functional>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
using namespace std;
using ll = long long;
using ull = unsigned long long;
using VI = vector<int>;
using VVI = vector<vector<int>>;
using VLL = vector<ll>;
using VVLL = vector<vector<ll>>;
using VB = vector<bool>;
using PII = pair<int, int>;
using PLL = pair<ll, ll>;
constexpr int INF = 1000000007;
constexpr ll INF_LL = 1'000'000'000'000'000'007;
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
#define newl '\n'

// loops rep(until) / rep(var, until) / rep(var, from, until) / repr (reversed order)
#define OVERLOAD3(_1, _2, _3, name, ...) name
#define rep(...) OVERLOAD3(__VA_ARGS__, REPEAT_FROM_UNTIL, REPEAT_UNTIL, REPEAT)(__VA_ARGS__)
#define REPEAT(times) REPEAT_CNT(_repeat, __COUNTER__, times)
#define REPEAT_CNT(_repeat, cnt, times) REPEAT_CNT_CAT(_repeat, cnt, times)
#define REPEAT_CNT_CAT(_repeat, cnt, times) REPEAT_FROM_UNTIL(_repeat ## cnt, 0, times)
#define REPEAT_UNTIL(name, times) REPEAT_FROM_UNTIL(name, 0, times)
#define REPEAT_FROM_UNTIL(name, from, until) for (int name = from, name ## __until = (until); name < name ## __until; name++)
#define repr(...) OVERLOAD3(__VA_ARGS__, REPR_FROM_UNTIL, REPR_UNTIL, REPEAT)(__VA_ARGS__)
#define REPR_UNTIL(name, times) REPR_FROM_UNTIL(name, 0, times)
#define REPR_FROM_UNTIL(name, from, until) for (int name = (until)-1, name ## __from = (from); name >= name ## __from; name--)

template <typename T, typename U>
bool chmin(T& var, U x) { if (var > x) { var = x; return true; } else return false; }
template <typename T, typename U>
bool chmax(T& var, U x) { if (var < x) { var = x; return true; } else return false; }
ll power(ll e, ll t, ll mod = INF_LL) {
  ll res = 1; for (; t; t >>= 1, (e *= e) %= mod) if (t & 1) (res *= e) %= mod; return res;
}
ll choose(ll n, int r) {
  chmin(r, n-r); if (r < 0) return 0; ll res = 1; rep(i, r) res *= n-i, res /= i+1; return res;
}
template <typename T, typename U> T divceil(T m, U d) { return (m + d - 1) / d; }
template <typename T> vector<T> make_v(size_t a, T b) { return vector<T>(a, b); }
template <typename... Ts> auto make_v(size_t a, Ts... ts) {
  return vector<decltype(make_v(ts...))>(a, make_v(ts...));
}

// debug stuff
#define repi(it, ds) for (auto it = ds.begin(); it != ds.end(); it++)
class DebugPrint { public: template <typename T> DebugPrint& operator <<(const T& v) {
#ifdef LOCAL
    cerr << v;
#endif
return *this; } } debugos; template <typename T> DebugPrint& operator<<(DebugPrint& os, const
vector<T>& vec) { os << "{"; for (int i = 0; i < vec.size(); i++) os << vec[i] << (i + 1 ==
vec.size() ? "" : ", "); os << "}"; return os; } template <typename T, typename U> DebugPrint&
operator<<(DebugPrint& os, map<T, U>& map_var) { os << "{"; repi(itr, map_var) { os << *itr;
itr++; if (itr != map_var.end()) os << ", "; itr--; } os << "}"; return os; } template <
typename T> DebugPrint& operator<<(DebugPrint& os, set<T>& set_var) { os << "{"; repi(itr,
set_var) { os << *itr; itr++; if (itr != set_var.end()) os << ", "; itr--; } os << "}"; return
os; } template <typename T, typename U> DebugPrint& operator<<(DebugPrint& os, const pair<T, U
>& p) { os << "(" << p.first << ", " << p.second << ")"; return os; } void dump_func() {
debugos << newl; } template <class Head, class... Tail> void dump_func(Head &&head, Tail &&...
tail) { debugos << head; if (sizeof...(Tail) > 0) { debugos << ", "; } dump_func(std::move(
tail)...); }
#define dump(...) debugos << "  " << string(#__VA_ARGS__) << ": " << "[" << to_string(__LINE__) \
<< ":" << __FUNCTION__ << "]" << newl << "    ", dump_func(__VA_ARGS__)
#pragma GCC diagnostic pop




int main() {
  int n, m; cin >> n >> m;
  VB red(n); red[0] = true;
  rep(m) {
    int x, y; cin >> x >> y;
    red[y-1] |= red[x-1];
  }
  cout << accumulate(all(red), 0) << newl;
}

Compiler output:
a.cc:1:9: warning: #pragma once in main file
    1 | #pragma once
      |         ^~~~
a.cc: In function 'int main()':
a.cc:88:14: error: no match for 'operator|=' (operand types are 'std::vector<bool>::reference' and 'std::vector<bool>::reference')
   88 |     red[y-1] |= red[x-1];
      |     ~~~~~~~~~^~~~~~~~~~~
In file included from /usr/include/c++/14/bits/memory_resource.h:38,
                 from /usr/include/c++/14/string:68,
                 from /usr/include/c++/14/bitset:52,
                 from a.cc:4:
/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/ios:44,
                 from /usr/include/c++/14/ostream:40,
                 from /usr/include/c++/14/iostream:41,
                 from a.cc:8:
/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
      |              ~~~~~~~~~~~~~~^~~