Language: C++

#include <iostream>
#include <vector>
#include <algorithm>
#include <map>
#include <cstring>
#include <math.h>
#include <bitset>
#include <queue>
#include <set>
#include <iomanip>
#include <assert.h>
#include <cstdio>

// #include<bits/stdc++.h>
using namespace std;
typedef long long ll;
constexpr long long int INFLL = 1001001001001001LL;
constexpr long long int infll = 1001001001001001LL;
constexpr int INF = 1000000007;
constexpr int inf = 1000000007;
const int mod = 1000000007;

inline bool chmin(ll &a, ll b)
{
    if (a > b)
    {
        a = b;
        return true;
    }
    return false;
}

inline bool chmax(ll &a, ll b)
{
    if (a < b)
    {
        a = b;
        return true;
    }
    return false;
}

template <typename T>
T seinomi(T a)
{
    if (a > 0)
    {
        return a;
    }
    else
    {
        return 0;
    }
}

//桁数取得
template <typename T>
T ketasuu(T num)
{
    return std::to_string(num).length();
}
//整数乗
ll llpow(ll a, ll n)
{
    if (n == 0)
    {
        return 1;
    }
    else
    {
        ll rep = a;
        for (ll i = 1; i < n; i++)
        {
            rep *= a;
        }
        return rep;
    }
}

template <class ForwardIt, class T>
void iota(ForwardIt first, ForwardIt last, T value)
{
    while (first != last)
    {
        *first++ = value;
        ++value;
    }
}

template <typename T>
T amarinasi(T a, T b)
{
    if (a % b == 0)
    {
        return a / b;
    }
    else if (a % b > 0)
    {
        return a / b + 1;
    }
    else
    {
        return a / b - 1;
    }
}

//小数点以下10桁テンプレート（main関数内の最初に貼付け）
//std::cout << std::fixed << std::setprecision(10);

//----------------------------------------------------------------
const double pi = 3.14159265358979323846264;
set<ll> se;
ll data;
int main()
{
    ll a, b, c;
    cin >> a >> b >> c;
    se.insert(a);
    se.insert(b);
    se.insert(c);
    data = se.size();
    cout << data << endl;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:121:5: error: reference to 'data' is ambiguous
  121 |     data = se.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:344:5: note: candidates are: 'template<class _Tp> constexpr const _Tp* std::data(initializer_list<_Tp>)'
  344 |     data(initializer_list<_Tp> __il) noexcept
      |     ^~~~
/usr/include/c++/14/bits/range_access.h:334:5: note:                 'template<class _Tp, long unsigned int _Nm> constexpr _Tp* std::data(_Tp (&)[_Nm])'
  334 |     data(_Tp (&__array)[_Nm]) noexcept
      |     ^~~~
/usr/include/c++/14/bits/range_access.h:323:5: note:                 'template<class _Container> constexpr decltype (__cont.data()) std::data(const _Container&)'
  323 |     data(const _Container& __cont) noexcept(noexcept(__cont.data()))
      |     ^~~~
/usr/include/c++/14/bits/range_access.h:312:5: note:                 'template<class _Container> constexpr decltype (__cont.data()) std::data(_Container&)'
  312 |     data(_Container& __cont) noexcept(noexcept(__cont.data()))
      |     ^~~~
a.cc:113:4: note:                 'll data'
  113 | ll data;
      |    ^~~~
a.cc:122:13: error: reference to 'data' is ambiguous
  122 |     cout << data << endl;
      |             ^~~~
/usr/include/c++/14/bits/range_access.h:344:5: note: candidates are: 'template<class _Tp> constexpr const _Tp* std::data(initializer_list<_Tp>)'
  344 |     data(initializer_list<_Tp> __il) noexcept
      |     ^~~~
/usr/include/c++/14/bits/range_access.h:334:5: note:                 'template<class _Tp, long unsigned int _Nm> constexpr _Tp* std::data(_Tp (&)[_Nm])'
  334 |     data(_Tp (&__array)[_Nm]) noexcept
      |     ^~~~
/usr/include/c++/14/bits/range_access.h:323:5: note:                 'template<class _Container> constexpr decltype (__cont.data()) std::data(const _Container&)'
  323 |     data(const _Container& __cont) noexcept(noexcept(__cont.data()))
      |     ^~~~
/usr/include/c++/14/bits/range_access.h:312:5: note:                 'template<class _Container> constexpr decltype (__cont.data()) std::data(_Container&)'
  312 |     data(_Container& __cont) noexcept(noexcept(__cont.data()))
      |     ^~~~
a.cc:113:4: note:                 'll data'
  113 | ll data;
      |    ^~~~