Language: C++

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using VI = vector<int>;
using VL = vector<ll>;
using VS = vector<string>;
template<class T> using PQ = priority_queue<T, vector<T>, greater<T>>;
#define FOR(i,a,n) for(int i=(a);i<(n);++i)
#define eFOR(i,a,n) for(int i=(a);i<=(n);++i)
#define rFOR(i,a,n) for(int i=(n)-1;i>=(a);--i)
#define erFOR(i,a,n) for(int i=(n);i>=(a);--i)
#define each(i, a) for(auto &i : a)
#define SORT(a) sort(a.begin(),a.end())
#define rSORT(a) sort(a.rbegin(),a.rend())
#define fSORT(a,f) sort(a.begin(),a.end(),f)
#define all(a) a.begin(),a.end()
#define out(y,x) ((y)<0||h<=(y)||(x)<0||w<=(x))
#define tp(a,i) get<i>(a)
#define line cout << "-----------------------------\n" 
#define ENDL(i,n) ((i) == (n) - 1 ? "\n" : " ")
#define stop system("pause")
constexpr ll INF = 1000000000;
constexpr ll LLINF = 1LL << 60;
constexpr ll mod = 1000000007;
constexpr ll MOD = 998244353;
constexpr ld eps = 1e-10;
constexpr ld pi = 3.1415926535897932;
template<class T>inline bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; }return false; }
template<class T>inline bool chmin(T& a, const T& b) { if (a > b) { a = b; return true; }return false; }
inline void init() { cin.tie(nullptr); cout.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); }
template<class T>inline istream& operator>>(istream& is, vector<T>& v) { for (auto& a : v)is >> a; return is; }
template<class T>inline istream& operator>>(istream& is, deque<T>& v) { for (auto& a : v)is >> a; return is; }
template<class T, class U>inline istream& operator>>(istream& is, pair<T, U>& p) { is >> p.first >> p.second; return is; }
template<class T>inline vector<T> vec(size_t a) { return vector<T>(a); }
template<class T>inline vector<T> defvec(T def, size_t a) { return vector<T>(a, def); }
template<class T, class... Ts>inline auto vec(size_t a, Ts... ts) { return vector<decltype(vec<T>(ts...))>(a, vec<T>(ts...)); }
template<class T, class... Ts>inline auto defvec(T def, size_t a, Ts... ts) { return vector<decltype(defvec<T>(def, ts...))>(a, defvec<T>(def, ts...)); }
template<class T>inline void print(const T& a) { cout << a << "\n"; }
template<class T, class... Ts>inline void print(const T& a, const Ts&... ts) { cout << a << " "; print(ts...); }
template<class T>inline void print(const vector<T>& v) { for (int i = 0; i < v.size(); ++i)cout << v[i] << (i == v.size() - 1 ? "\n" : " "); }
template<class T>inline void print(const vector<vector<T>>& v) { for (auto& a : v)print(a); }
inline string reversed(const string& s) { string t = s; reverse(all(t)); return t; }

template<class Op> class segtree {
	using T = typename Op::T;
	int len, n;
	vector<T> dat;
public:
	segtree(const int& _n) : n(_n) {
		for (len = 1; len < n; len <<= 1);
		dat.resize(len << 1, Op::unit);
	}
	segtree(const vector<T> v) : n(v.size()) {
		for (len = 1; len < n; len <<= 1);
		dat.resize(len << 1, Op::unit);
		for (int i = 0; i < n; ++i)dat[i + len] = v[i];
		for (int i = len - 1; 0 < i; --i)
			dat[i] = Op::merge(dat[i << 1], dat[i << 1 | 1]);
	}
	inline void update(int idx, T a) {
		idx += len;
		dat[idx] = Op::update(dat[idx], a);
		for (idx >>= 1; 0 < idx; idx >>= 1)
			dat[idx] = Op::merge(dat[idx << 1], dat[idx << 1 | 1]);
	}
	inline T value(int l, int r) {
		T vl = Op::unit, vr = Op::unit;
		for (l += len, r += len; l < r; l >>= 1, r >>= 1) {
			if (l & 1)vl = Op::merge(vl, dat[l++]);
			if (r & 1)vr = Op::merge(dat[--r], vr);
		}
		return Op::merge(vl, vr);
	}
	inline T operator[](int idx) { return dat[idx + len]; }
};

template<class Type>struct int_gcd {
	using T = Type;
	T unit = 0;
	T merge(T vl, T vr) { return gcd(vl, vr); }
	T update(T vl, T vr) { return vr; }
};

int main() {
    init();

	int n; cin >> n;
	VI a(n); cin >> a;

	segtree<int_gcd<int>> seg(a);
	int ans = max(seg.value(0, n - 1), seg.value(1, n));
	FOR(i, 1, n - 1)
		chmax(ans, gcd(seg.value(0, i), seg.value(i + 1, n)));
	print(ans);

    return 0;
}

Compiler output:
a.cc: In instantiation of 'segtree<Op>::segtree(std::vector<typename Op::T>) [with Op = int_gcd<int>; typename Op::T = int]':
a.cc:91:29:   required from here
   91 |         segtree<int_gcd<int>> seg(a);
      |                                    ^
a.cc:56:42: error: invalid use of non-static data member 'int_gcd<int>::unit'
   56 |                 dat.resize(len << 1, Op::unit);
      |                                          ^~~~
a.cc:80:11: note: declared here
   80 |         T unit = 0;
      |           ^~~~
a.cc:59:43: error: cannot call member function 'int_gcd<Type>::T int_gcd<Type>::merge(T, T) [with Type = int; T = int]' without object
   59 |                         dat[i] = Op::merge(dat[i << 1], dat[i << 1 | 1]);
      |                                  ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a.cc: In instantiation of 'segtree<Op>::T segtree<Op>::value(int, int) [with Op = int_gcd<int>; T = int]':
a.cc:92:25:   required from here
   92 |         int ans = max(seg.value(0, n - 1), seg.value(1, n));
      |                       ~~~~~~~~~^~~~~~~~~~
a.cc:68:28: error: invalid use of non-static data member 'int_gcd<int>::unit'
   68 |                 T vl = Op::unit, vr = Op::unit;
      |                            ^~~~
a.cc:80:11: note: declared here
   80 |         T unit = 0;
      |           ^~~~
a.cc:68:43: error: invalid use of non-static data member 'int_gcd<int>::unit'
   68 |                 T vl = Op::unit, vr = Op::unit;
      |                                           ^~~~
a.cc:80:11: note: declared here
   80 |         T unit = 0;
      |           ^~~~
a.cc:70:49: error: cannot call member function 'int_gcd<Type>::T int_gcd<Type>::merge(T, T) [with Type = int; T = int]' without object
   70 |                         if (l & 1)vl = Op::merge(vl, dat[l++]);
      |                                        ~~~~~~~~~^~~~~~~~~~~~~~
a.cc:71:49: error: cannot call member function 'int_gcd<Type>::T int_gcd<Type>::merge(T, T) [with Type = int; T = int]' without object
   71 |                         if (r & 1)vr = Op::merge(dat[--r], vr);
      |                                        ~~~~~~~~~^~~~~~~~~~~~~~
a.cc:73:33: error: cannot call member function 'int_gcd<Type>::T int_gcd<Type>::merge(T, T) [with Type = int; T = int]' without object
   73 |                 return Op::merge(vl, vr);
      |                        ~~~~~~~~~^~~~~~~~