Language: C++

#include <bits/stdc++.h>
using namespace std;

int main() {
  string alphabet;
  cin >> alphabet;
  if (alphabet==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;
  }
  else {
    cout << a;
  }
}

Compiler output:
a.cc: In function 'int main()':
a.cc:7:17: error: 'A' was not declared in this scope
    7 |   if (alphabet==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:7:20: error: 'B' was not declared in this scope
    7 |   if (alphabet==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:7:23: error: 'C' was not declared in this scope
    7 |   if (alphabet==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:7:26: error: 'D' was not declared in this scope
    7 |   if (alphabet==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:7:29: error: 'E' was not declared in this scope
    7 |   if (alphabet==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:7:32: error: 'F' was not declared in this scope
    7 |   if (alphabet==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:7:35: error: 'G' was not declared in this scope
    7 |   if (alphabet==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:7:38: error: 'H' was not declared in this scope
    7 |   if (alphabet==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:7:41: error: 'I' was not declared in this scope
    7 |   if (alphabet==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:7:44: error: 'J' was not declared in this scope
    7 |   if (alphabet==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:7:47: error: 'K' was not declared in this scope
    7 |   if (alphabet==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:7:50: error: 'L' was not declared in this scope
    7 |   if (alphabet==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:7:53: error: 'M' was not declared in this scope
    7 |   if (alphabet==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:7:56: error: 'N' was not declared in this scope
    7 |   if (alphabet==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:7:59: error: 'O' was not declared in this scope
    7 |   if (alphabet==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:7:62: error: 'P' was not declared in this scope
    7 |   if (alphabet==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:7:65: error: 'Q' was not declared in this scope
    7 |   if (alphabet==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:7:68: error: 'R' was not declared in this scope
    7 |   if (alphabet==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:7:71: error: 'S' was not declared in this scope
    7 |   if (alphabet==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:7:74: error: 'T' was not declared in this scope
    7 |   if (alphabet==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:7:77: error: 'U' was not declared in this scope
    7 |   if (alphabet==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:7:80: error: 'V' was not declared in this scope
    7 |   if (alphabet==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:7:83: error: 'W' was not declared in this scope
    7 |   if (alphabet==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:7:86: error: 'X' was not declared in this scope
    7 |   if (alphabet==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:7:89: error: 'Y' was not declared in this scope
    7 |   if (alphabet==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:7:92: error: 'Z' was not declared in this scope
    7 |   if (alphabet==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:11:13: error: 'a' was not declared in this scope
   11 |     cout << a;
      |             ^