Language: C++

#pragma warning(disable:4996)
#include <bits/stdc++.h>

using namespace std;

//macro
#define FOR(i,a,b) for(ll i=(ll)(a);i<(ll)(b);i++)
#define REP(i,n) FOR(i,0,n)
#define ALL(a) (a).begin(),(a).end()
#define EPS (1e-8)
#define equals(a,b)(fabs((a)-(b))<EPS)

using ll = long long;
using ull = unsigned long long;
using vb = vector<bool>;
using vi = vector<int>;
using vll = vector<ll>;
using vs = vector<string>;
using vvb = vector<vb>;
using vvi = vector<vi>;
using vvll = vector<vll>;
//func
template<typename A, typename T>void assign2d(A&arr, ll H, ll W, T t) { arr.resize(H); REP(i, H) arr[i].assign(W, t); }
template<class T> ostream &operator<<(ostream &os, const vector<T> &v) { int n = v.size(); REP(i, n)os << v[i] << (i == n - 1 ? "" : " "); return os; }
template <class T = int> inline T IN() { T x; cin >> x; return x; }
template<class T> inline void OUT(const T &x) { cout << x << "\n"; }
void YESNO(bool c) { OUT(c ? "YES" : "NO"); };
void YesNo(bool c) { OUT(c ? "Yes" : "No"); };
struct pre_ { pre_() { cin.tie(nullptr); ios::sync_with_stdio(false); /*cout << fixed << setprecision(6);*/ } } pre__;

int main() {
	vs S(3);
	REP(i, 3)cin >> S[i];
	string ans;
	REP(i, 3) {
		ans += S[i][0];
	}
	transform(ans.begin(), ans.end(), ans.begin(), toupper);
	OUT(ans);
}

Compiler output:
a.cc: In function 'int main()':
a.cc:38:18: error: no matching function for call to 'transform(std::__cxx11::basic_string<char>::iterator, std::__cxx11::basic_string<char>::iterator, std::__cxx11::basic_string<char>::iterator, <unresolved overloaded function type>)'
   38 |         transform(ans.begin(), ans.end(), ans.begin(), toupper);
      |         ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/14/algorithm:61,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51,
                 from a.cc:2:
/usr/include/c++/14/bits/stl_algo.h:4209:5: note: candidate: 'template<class _IIter, class _OIter, class _UnaryOperation> _OIter std::transform(_IIter, _IIter, _OIter, _UnaryOperation)'
 4209 |     transform(_InputIterator __first, _InputIterator __last,
      |     ^~~~~~~~~
/usr/include/c++/14/bits/stl_algo.h:4209:5: note:   template argument deduction/substitution failed:
a.cc:38:18: note:   couldn't deduce template parameter '_UnaryOperation'
   38 |         transform(ans.begin(), ans.end(), ans.begin(), toupper);
      |         ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algo.h:4247:5: note: candidate: 'template<class _IIter1, class _IIter2, class _OIter, class _BinaryOperation> _OIter std::transform(_IIter1, _IIter1, _IIter2, _OIter, _BinaryOperation)'
 4247 |     transform(_InputIterator1 __first1, _InputIterator1 __last1,
      |     ^~~~~~~~~
/usr/include/c++/14/bits/stl_algo.h:4247:5: note:   candidate expects 5 arguments, 4 provided
In file included from /usr/include/c++/14/algorithm:86:
/usr/include/c++/14/pstl/glue_algorithm_defs.h:156:1: note: candidate: 'template<class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _UnaryOperation> __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2> std::transform(_ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _UnaryOperation)'
  156 | transform(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last, _ForwardIterator2 __result,
      | ^~~~~~~~~
/usr/include/c++/14/pstl/glue_algorithm_defs.h:156:1: note:   candidate expects 5 arguments, 4 provided
/usr/include/c++/14/pstl/glue_algorithm_defs.h:162:1: note: candidate: 'template<class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _ForwardIterator, class _BinaryOperation> __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2> std::transform(_ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator, _BinaryOperation)'
  162 | transform(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2,
      | ^~~~~~~~~
/usr/include/c++/14/pstl/glue_algorithm_defs.h:162:1: note:   candidate expects 6 arguments, 4 provided