Language: C++

constexpr bool isDebugMode = false;
#include <bits/stdc++.h>
#if __has_include(<atcoder/all>)
    #include <atcoder/all>
    using namespace atcoder;
#endif
using namespace std;
typedef long long ll;
typedef pair<long long, long long> P;
struct edge{long long to,cost;};
const int inf = 1 << 27;
const long long INF = 1LL << 60;
const int COMBMAX = 1001001;
const long long MOD = 1000000007;
const long long dy[] = {-1, 0, 0, 1};
const long long dx[] = {0, -1, 1, 0};
string abc = "abcdefghijklmnopqrstuvwxyz";
#define rep(i, n) for(int i = 0; i < (n); ++i)
#define eachdo(v, e) for (const auto &e : (v))
#define all(v) (v).begin(), (v).end()
#define lower_index(v, e) (long long)distance((v).begin(), lower_bound((v).begin(), (v).end(), e))
#define upper_index(v, e) (long long)distance((v).begin(), upper_bound((v).begin(), (v).end(), e))
long long mpow(long long a, long long n, long long mod = MOD){long long res = 1; while(n > 0){if(n & 1)res = res * a % mod; a = a * a % mod; n >>= 1;} return res;}
void pt(){cout << endl; return;}
template<class Head> void pt(Head&& head){cout << head; pt(); return;}
template<class Head, class... Tail> void pt(Head&& head, Tail&&... tail){cout << head << " "; pt(forward<Tail>(tail)...);}
void dpt(){if(!isDebugMode) return; cout << endl; return;}
template<class Head> void dpt(Head&& head){if(!isDebugMode) return; cout << head; pt(); return;}
template<class Head, class... Tail> void dpt(Head&& head, Tail&&... tail){if(!isDebugMode) return; cout << head << " "; pt(forward<Tail>(tail)...);}
template<class T> void debug(T v){if(!isDebugMode) return; rep(i, v.size()) cout << v[i] << " " ; cout << endl;}
template<class T> void debug2(T v){if(!isDebugMode) return; rep(i, v.size()){rep(j, v[i].size()) cout << v[i][j] << " " ; cout << endl;}}
template<class T1, class T2> inline bool chmin(T1 &a, T2 b){if(a > b){a = b; return true;} return false;}
template<class T1, class T2> inline bool chmax(T1 &a, T2 b){if(a < b){a = b; return true;} return false;}
template<class T1, class T2> long long recgcd(T1 a, T2 b){return a % b ? recgcd(b , a % b) : b;}
bool valid(long long H, long long W, long long h, long long w) { return 0 <= h && h < H && 0 <= w && w < W; }

int main(){
    ll N, Q; cin >> N >> Q;
    vector<ll> a(N);
    rep(i, N) cin >> a[i];
    vector<ll> ans;
    segtree<ll ,op, e> seg(a);
    rep(i, Q){
        ll q; cin >> q;
        if(q == 1){
            ll x, v; cin >> x >> v;
            seg.set(--x, v);
        }else if(q == 2){
            ll x, v; cin >> x >> v;
            ans.push_back(seg.prod(--x, v));
        }else{
            ll x, v; cin >> x >> v;
            gv = v;
            ans.push_back(seg.max_right<f>(--x) + 1);
        }
    }
    debug(ans);
    return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:42:5: error: 'segtree' was not declared in this scope
   42 |     segtree<ll ,op, e> seg(a);
      |     ^~~~~~~
a.cc:42:16: error: expected primary-expression before ',' token
   42 |     segtree<ll ,op, e> seg(a);
      |                ^
a.cc:42:17: error: 'op' was not declared in this scope
   42 |     segtree<ll ,op, e> seg(a);
      |                 ^~
a.cc:42:21: error: 'e' was not declared in this scope
   42 |     segtree<ll ,op, e> seg(a);
      |                     ^
a.cc:42:24: error: 'seg' was not declared in this scope
   42 |     segtree<ll ,op, e> seg(a);
      |                        ^~~
a.cc:53:13: error: 'gv' was not declared in this scope; did you mean 'v'?
   53 |             gv = v;
      |             ^~
      |             v
a.cc:54:41: error: 'f' was not declared in this scope
   54 |             ans.push_back(seg.max_right<f>(--x) + 1);
      |                                         ^