Language: C++

#include<bits/stdc++.h>
using namespace std;

// for loop
#define rep(i,a,n) for(int i=(a);i<(n);++i)
#define repe(i,a,n) for(int i=(a);i<=(n);++i)
#define rrep(i,n,a) for(int i=(n);n>(a);--i)
#define rrepe(i,n,a) for(int i=(n);n>=(a);--i)

// abbreviati
#define PB(v) push_back(v)
#define MP(A,B) make_pair(A,B)
#define F first
#define S second
#define all(v) v.begin(),v.end()
#define ENDL << "\n"

using ll = long long;
using P = pair<int,int>;
using itn = int;

// constant
const int INF = 1<<30;
const ll INFLL = 1LL << 60;
const ll MOD = 1000000007;
const long double PI =(acos(-1));

/***Snippet***************************************************
ifelse, isprime, torad, todeg, lcm, bfs,chmin,chmax
**************************************************************/

// adjacency list
vector<int> to[100005];

// Graph
using Graph = vector<vector<int>>;

// Grid
int H,W;
vector<vector<int>> field;

// 4方向への移動ベクトル
const int dx[4]={1,0,-1,0};
const int dy[4]={0,1,0,-1};


int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    vector<int> a(4);
    rep(i,0,4) cin >> a[i];
    ll ans=-INF;
    // ans=(abs(a*c),min(abs(a*d),min(abs(b*c),abs(b*d))));
    // if(ans==abs(a*c)) {cout << a*c ENDL; return 0 }
    // if(ans==abs(a*d)) cout << a*d ENDL;
    // if(ans==abs(b*c)) cout << b*c ENDL;
    // if(ans==abs(b*d)) cout << b*d ENDL;

    rep(i,0,2){
        rep(j,2,4){
            ans=max(ans,a[i]*a[j]);
        }
    }
    
    cout << ans ENDL;
    
    return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:61:20: error: no matching function for call to 'max(ll&, __gnu_cxx::__alloc_traits<std::allocator<int>, int>::value_type)'
   61 |             ans=max(ans,a[i]*a[j]);
      |                 ~~~^~~~~~~~~~~~~~~
In file included from /usr/include/c++/14/algorithm:60,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_algobase.h:257:5: note: candidate: 'template<class _Tp> constexpr const _Tp& std::max(const _Tp&, const _Tp&)'
  257 |     max(const _Tp& __a, const _Tp& __b)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:257:5: note:   template argument deduction/substitution failed:
a.cc:61:20: note:   deduced conflicting types for parameter 'const _Tp' ('long long int' and '__gnu_cxx::__alloc_traits<std::allocator<int>, int>::value_type' {aka 'int'})
   61 |             ans=max(ans,a[i]*a[j]);
      |                 ~~~^~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algobase.h:303:5: note: candidate: 'template<class _Tp, class _Compare> constexpr const _Tp& std::max(const _Tp&, const _Tp&, _Compare)'
  303 |     max(const _Tp& __a, const _Tp& __b, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:303:5: note:   candidate expects 3 arguments, 2 provided
In file included from /usr/include/c++/14/algorithm:61:
/usr/include/c++/14/bits/stl_algo.h:5706:5: note: candidate: 'template<class _Tp> constexpr _Tp std::max(initializer_list<_Tp>)'
 5706 |     max(initializer_list<_Tp> __l)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5706:5: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_algo.h:5716:5: note: candidate: 'template<class _Tp, class _Compare> constexpr _Tp std::max(initializer_list<_Tp>, _Compare)'
 5716 |     max(initializer_list<_Tp> __l, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5716:5: note:   template argument deduction/substitution failed:
a.cc:61:20: note:   mismatched types 'std::initializer_list<_Tp>' and 'long long int'
   61 |             ans=max(ans,a[i]*a[j]);
      |                 ~~~^~~~~~~~~~~~~~~