Language: C++

#include<bits/stdc++.h>
#define ll long long int
#define FOR(i,a,b) for(ll i=(ll)(a);i<(ll)(b);i++)
#define REP(i,n) FOR(i,0,n)
#define REP1(i,n) FOR(i,1,n)
#define MAX 500000
#define SENTINEL 2000000000
using namespace std;

int L[MAX/2 + 2], R[MAX/2 + 2];
int count;

void merge(int A[], int n, int left, int mid, int right) {
  int n1 = mid - left;
  int n2 = right - mid;
  REP(i, n1) L[i] = A[left + i];
  REP(i, n2) R[i] = A[mid + i];
  L[n1] = R[n2] = SENTINEL;
  int i = 0, j = 0;
  FOR(k, left, right) {
    count++;
    if ( L[i] <= R[j] ) {
      A[k] = L[i++];
    } else {
      A[k] = R[j++];
    }
  }
}

void mergeSort(int A[], int n, int left, int right) {
  if (left+1 < right) {
    int mid = (left + right) / 2;
    mergeSort(A, n, left, mid);
    mergeSort(A, n, mid, right);
    merge(A, n, left, mid, right);
  }
}

int main() {
  int A[MAX], n, i;
  count = 0;

  cin >> n;
  REP(i, n) {
    cin >> A[i];
  }

  mergeSort(A, n, 0, n);

  REP(i, n) {
    if ( i ) cout << " ";
    cout << A[i];
  }
  cout << endl;

  cout << count << endl;

  return 0;
}

Compiler output:
a.cc: In function 'void merge(int*, int, int, int, int)':
a.cc:21:5: error: reference to 'count' is ambiguous
   21 |     count++;
      |     ^~~~~
In file included from /usr/include/c++/14/algorithm:86,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51,
                 from a.cc:1:
/usr/include/c++/14/pstl/glue_algorithm_defs.h:101:1: note: candidates are: 'template<class _ExecutionPolicy, class _ForwardIterator, class _Tp> __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, typename std::iterator_traits<_II>::difference_type> std::count(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, const _Tp&)'
  101 | count(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, const _Tp& __value);
      | ^~~~~
In file included from /usr/include/c++/14/algorithm:61:
/usr/include/c++/14/bits/stl_algo.h:4025:5: note:                 'template<class _IIter, class _Tp> typename std::iterator_traits< <template-parameter-1-1> >::difference_type std::count(_IIter, _IIter, const _Tp&)'
 4025 |     count(_InputIterator __first, _InputIterator __last, const _Tp& __value)
      |     ^~~~~
a.cc:11:5: note:                 'int count'
   11 | int count;
      |     ^~~~~
a.cc: In function 'int main()':
a.cc:41:3: error: reference to 'count' is ambiguous
   41 |   count = 0;
      |   ^~~~~
/usr/include/c++/14/pstl/glue_algorithm_defs.h:101:1: note: candidates are: 'template<class _ExecutionPolicy, class _ForwardIterator, class _Tp> __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, typename std::iterator_traits<_II>::difference_type> std::count(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, const _Tp&)'
  101 | count(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, const _Tp& __value);
      | ^~~~~
/usr/include/c++/14/bits/stl_algo.h:4025:5: note:                 'template<class _IIter, class _Tp> typename std::iterator_traits< <template-parameter-1-1> >::difference_type std::count(_IIter, _IIter, const _Tp&)'
 4025 |     count(_InputIterator __first, _InputIterator __last, const _Tp& __value)
      |     ^~~~~
a.cc:11:5: note:                 'int count'
   11 | int count;
      |     ^~~~~
a.cc:56:11: error: reference to 'count' is ambiguous
   56 |   cout << count << endl;
      |           ^~~~~
/usr/include/c++/14/pstl/glue_algorithm_defs.h:101:1: note: candidates are: 'template<class _ExecutionPolicy, class _ForwardIterator, class _Tp> __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, typename std::iterator_traits<_II>::difference_type> std::count(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, const _Tp&)'
  101 | count(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, const _Tp& __value);
      | ^~~~~
/usr/include/c++/14/bits/stl_algo.h:4025:5: note:                 'template<class _IIter, class _Tp> typename std::iterator_traits< <template-parameter-1-1> >::difference_type std::count(_IIter, _IIter, const _Tp&)'
 4025 |     count(_InputIterator __first, _InputIterator __last, const _Tp& __value)
      |     ^~~~~
a.cc:11:5: note:                 'int count'
   11 | int count;
      |     ^~~~~