Language: C++

#include <iostream>
#include <sstream>
#include <vector>
#include <algorithm>
#include <numeric>
#include <cmath>
#include <utility>
#include <map>
#include <list>
#include <climits>
#include <set>
#include <deque>
#include <queue>
#include <stack>
#include <unordered_map>
#include <unordered_set>
#include <iomanip>
#include <bitset>
#include <string>

// #define cerr if(false)cerr
#define watch(x) cerr << "> " << #x << ": " << x << "\n";

using namespace std;

template <typename T>
std::ostream &operator <<(std::ostream &out, vector<T> &v) {
	for (typename vector<T>::size_type i = 0; i < v.size(); ++i)
		out << v[i] << " ";
	out << "\n";
    return out;
}
template <typename T, typename N>
std::ostream &operator <<(std::ostream &out, vector<pair<T, N> > &v) {
	for (size_t i = 0; i < v.size(); ++i)
		out << "(" << v[i].first << ", " << v[i].second << ") ";
	out << "\n";
    return out;
}
template <typename T>
std::ostream &operator <<(std::ostream &out, vector<vector<T> > &v) {
	for (size_t i = 0; i < v.size(); ++i) {
		for (size_t j = 0; j < v[i].size(); ++j) {
			out << v[i][j] << " ";
		}
		out << "\n";
	}
   	return out;
}

vector<set<int> > graph;
vector<int> visited;
int size;

void dfs(int u) {
	if (!visited[u]) {
		visited[u] = 1;
		++size;
	}

	for (auto v : graph[u]) {
		if (!visited[v]) {
			dfs(v);
		}
	}
}

int main() {

	std::ios::sync_with_stdio(false);
	cin.tie(0);
	cout.tie(0);

	int n, m;
	cin >> n >> m;

	graph = vector<set<int> > (n + 1, set<int> ());
	int u, v;
	for (int i = 0; i < m; ++i) {	
		cin >> u >> v;
		graph[u].insert(v);
		graph[v].insert(u);
	}

	visited = vector<int> (n + 1);
	int groups = 0;
	for (int i = 1; i <= n; ++i) {
		size = 0;
		if (!visited[i]) {
			dfs(i);
		}

		groups = max(groups, size);
	}

	cout << groups << "\n";

	return 0;

}

Compiler output:
a.cc: In function 'void dfs(int)':
a.cc:58:19: error: reference to 'size' is ambiguous
   58 |                 ++size;
      |                   ^~~~
In file included from /usr/include/c++/14/string:53,
                 from /usr/include/c++/14/bits/locale_classes.h:40,
                 from /usr/include/c++/14/bits/ios_base.h:41,
                 from /usr/include/c++/14/ios:44,
                 from /usr/include/c++/14/ostream:40,
                 from /usr/include/c++/14/iostream:41,
                 from a.cc:1:
/usr/include/c++/14/bits/range_access.h:272:5: note: candidates are: 'template<class _Tp, long unsigned int _Nm> constexpr std::size_t std::size(const _Tp (&)[_Nm])'
  272 |     size(const _Tp (&)[_Nm]) noexcept
      |     ^~~~
/usr/include/c++/14/bits/range_access.h:262:5: note:                 'template<class _Container> constexpr decltype (__cont.size()) std::size(const _Container&)'
  262 |     size(const _Container& __cont) noexcept(noexcept(__cont.size()))
      |     ^~~~
a.cc:53:5: note:                 'int size'
   53 | int size;
      |     ^~~~
a.cc: In function 'int main()':
a.cc:88:17: error: reference to 'size' is ambiguous
   88 |                 size = 0;
      |                 ^~~~
/usr/include/c++/14/bits/range_access.h:272:5: note: candidates are: 'template<class _Tp, long unsigned int _Nm> constexpr std::size_t std::size(const _Tp (&)[_Nm])'
  272 |     size(const _Tp (&)[_Nm]) noexcept
      |     ^~~~
/usr/include/c++/14/bits/range_access.h:262:5: note:                 'template<class _Container> constexpr decltype (__cont.size()) std::size(const _Container&)'
  262 |     size(const _Container& __cont) noexcept(noexcept(__cont.size()))
      |     ^~~~
a.cc:53:5: note:                 'int size'
   53 | int size;
      |     ^~~~
a.cc:93:38: error: reference to 'size' is ambiguous
   93 |                 groups = max(groups, size);
      |                                      ^~~~
/usr/include/c++/14/bits/range_access.h:272:5: note: candidates are: 'template<class _Tp, long unsigned int _Nm> constexpr std::size_t std::size(const _Tp (&)[_Nm])'
  272 |     size(const _Tp (&)[_Nm]) noexcept
      |     ^~~~
/usr/include/c++/14/bits/range_access.h:262:5: note:                 'template<class _Container> constexpr decltype (__cont.size()) std::size(const _Container&)'
  262 |     size(const _Container& __cont) noexcept(noexcept(__cont.size()))
      |     ^~~~
a.cc:53:5: note:                 'int size'
   53 | int size;
      |     ^~~~