Language: C++

#include <bits/stdc++.h> 
#define rep(i,n) for (int i=0;i<n;i++)
using namespace std;
using ll = long long;
using P = pair<int,int>;

int main() {
  string x;
  cin>>x;
  if(x==a||b||c||d||e||f||g||h||i||j||k||l||m||n||o||p||q||r||s||t||u||v||w||x||y||z){
    cout<<"A"<<endl;
  }
  else{
    cout<<"a"<<endl;
  }
}


Compiler output:
a.cc: In function 'int main()':
a.cc:10:9: error: 'a' was not declared in this scope
   10 |   if(x==a||b||c||d||e||f||g||h||i||j||k||l||m||n||o||p||q||r||s||t||u||v||w||x||y||z){
      |         ^
a.cc:10:12: error: 'b' was not declared in this scope
   10 |   if(x==a||b||c||d||e||f||g||h||i||j||k||l||m||n||o||p||q||r||s||t||u||v||w||x||y||z){
      |            ^
a.cc:10:15: error: 'c' was not declared in this scope
   10 |   if(x==a||b||c||d||e||f||g||h||i||j||k||l||m||n||o||p||q||r||s||t||u||v||w||x||y||z){
      |               ^
a.cc:10:18: error: 'd' was not declared in this scope
   10 |   if(x==a||b||c||d||e||f||g||h||i||j||k||l||m||n||o||p||q||r||s||t||u||v||w||x||y||z){
      |                  ^
a.cc:10:21: error: 'e' was not declared in this scope
   10 |   if(x==a||b||c||d||e||f||g||h||i||j||k||l||m||n||o||p||q||r||s||t||u||v||w||x||y||z){
      |                     ^
a.cc:10:24: error: 'f' was not declared in this scope
   10 |   if(x==a||b||c||d||e||f||g||h||i||j||k||l||m||n||o||p||q||r||s||t||u||v||w||x||y||z){
      |                        ^
a.cc:10:27: error: 'g' was not declared in this scope
   10 |   if(x==a||b||c||d||e||f||g||h||i||j||k||l||m||n||o||p||q||r||s||t||u||v||w||x||y||z){
      |                           ^
a.cc:10:30: error: 'h' was not declared in this scope
   10 |   if(x==a||b||c||d||e||f||g||h||i||j||k||l||m||n||o||p||q||r||s||t||u||v||w||x||y||z){
      |                              ^
a.cc:10:33: error: 'i' was not declared in this scope
   10 |   if(x==a||b||c||d||e||f||g||h||i||j||k||l||m||n||o||p||q||r||s||t||u||v||w||x||y||z){
      |                                 ^
a.cc:10:36: error: 'j' was not declared in this scope
   10 |   if(x==a||b||c||d||e||f||g||h||i||j||k||l||m||n||o||p||q||r||s||t||u||v||w||x||y||z){
      |                                    ^
a.cc:10:39: error: 'k' was not declared in this scope
   10 |   if(x==a||b||c||d||e||f||g||h||i||j||k||l||m||n||o||p||q||r||s||t||u||v||w||x||y||z){
      |                                       ^
a.cc:10:42: error: 'l' was not declared in this scope
   10 |   if(x==a||b||c||d||e||f||g||h||i||j||k||l||m||n||o||p||q||r||s||t||u||v||w||x||y||z){
      |                                          ^
a.cc:10:45: error: 'm' was not declared in this scope
   10 |   if(x==a||b||c||d||e||f||g||h||i||j||k||l||m||n||o||p||q||r||s||t||u||v||w||x||y||z){
      |                                             ^
a.cc:10:48: error: 'n' was not declared in this scope
   10 |   if(x==a||b||c||d||e||f||g||h||i||j||k||l||m||n||o||p||q||r||s||t||u||v||w||x||y||z){
      |                                                ^
a.cc:10:51: error: 'o' was not declared in this scope
   10 |   if(x==a||b||c||d||e||f||g||h||i||j||k||l||m||n||o||p||q||r||s||t||u||v||w||x||y||z){
      |                                                   ^
a.cc:10:54: error: 'p' was not declared in this scope
   10 |   if(x==a||b||c||d||e||f||g||h||i||j||k||l||m||n||o||p||q||r||s||t||u||v||w||x||y||z){
      |                                                      ^
a.cc:10:57: error: 'q' was not declared in this scope
   10 |   if(x==a||b||c||d||e||f||g||h||i||j||k||l||m||n||o||p||q||r||s||t||u||v||w||x||y||z){
      |                                                         ^
a.cc:10:60: error: 'r' was not declared in this scope
   10 |   if(x==a||b||c||d||e||f||g||h||i||j||k||l||m||n||o||p||q||r||s||t||u||v||w||x||y||z){
      |                                                            ^
a.cc:10:63: error: 's' was not declared in this scope
   10 |   if(x==a||b||c||d||e||f||g||h||i||j||k||l||m||n||o||p||q||r||s||t||u||v||w||x||y||z){
      |                                                               ^
a.cc:10:66: error: 't' was not declared in this scope
   10 |   if(x==a||b||c||d||e||f||g||h||i||j||k||l||m||n||o||p||q||r||s||t||u||v||w||x||y||z){
      |                                                                  ^
a.cc:10:69: error: 'u' was not declared in this scope
   10 |   if(x==a||b||c||d||e||f||g||h||i||j||k||l||m||n||o||p||q||r||s||t||u||v||w||x||y||z){
      |                                                                     ^
a.cc:10:72: error: 'v' was not declared in this scope
   10 |   if(x==a||b||c||d||e||f||g||h||i||j||k||l||m||n||o||p||q||r||s||t||u||v||w||x||y||z){
      |                                                                        ^
a.cc:10:75: error: 'w' was not declared in this scope
   10 |   if(x==a||b||c||d||e||f||g||h||i||j||k||l||m||n||o||p||q||r||s||t||u||v||w||x||y||z){
      |                                                                           ^
a.cc:10:81: error: 'y' was not declared in this scope
   10 |   if(x==a||b||c||d||e||f||g||h||i||j||k||l||m||n||o||p||q||r||s||t||u||v||w||x||y||z){
      |                                                                                 ^
a.cc:10:84: error: 'z' was not declared in this scope
   10 |   if(x==a||b||c||d||e||f||g||h||i||j||k||l||m||n||o||p||q||r||s||t||u||v||w||x||y||z){
      |                                                                                    ^