Language: C++

#include <iostream>
using namespace std;
int main() {
	char a;
	if (a = Q || W || E || R || T || Y || U || I || O || P || L || K || J || H || G || F || D || S || A || Z || X || C || V || B || N || M)
		cout << 'A ' << endl;
	else
		cout << 'a' << endl;
	return 0;
}

Compiler output:
a.cc:6:25: warning: multi-character character constant [-Wmultichar]
    6 |                 cout << 'A ' << endl;
      |                         ^~~~
a.cc: In function 'int main()':
a.cc:5:17: error: 'Q' was not declared in this scope
    5 |         if (a = Q || W || E || R || T || Y || U || I || O || P || L || K || J || H || G || F || D || S || A || Z || X || C || V || B || N || M)
      |                 ^
a.cc:5:22: error: 'W' was not declared in this scope
    5 |         if (a = Q || W || E || R || T || Y || U || I || O || P || L || K || J || H || G || F || D || S || A || Z || X || C || V || B || N || M)
      |                      ^
a.cc:5:27: error: 'E' was not declared in this scope
    5 |         if (a = Q || W || E || R || T || Y || U || I || O || P || L || K || J || H || G || F || D || S || A || Z || X || C || V || B || N || M)
      |                           ^
a.cc:5:32: error: 'R' was not declared in this scope
    5 |         if (a = Q || W || E || R || T || Y || U || I || O || P || L || K || J || H || G || F || D || S || A || Z || X || C || V || B || N || M)
      |                                ^
a.cc:5:37: error: 'T' was not declared in this scope
    5 |         if (a = Q || W || E || R || T || Y || U || I || O || P || L || K || J || H || G || F || D || S || A || Z || X || C || V || B || N || M)
      |                                     ^
a.cc:5:42: error: 'Y' was not declared in this scope
    5 |         if (a = Q || W || E || R || T || Y || U || I || O || P || L || K || J || H || G || F || D || S || A || Z || X || C || V || B || N || M)
      |                                          ^
a.cc:5:47: error: 'U' was not declared in this scope
    5 |         if (a = Q || W || E || R || T || Y || U || I || O || P || L || K || J || H || G || F || D || S || A || Z || X || C || V || B || N || M)
      |                                               ^
a.cc:5:52: error: 'I' was not declared in this scope
    5 |         if (a = Q || W || E || R || T || Y || U || I || O || P || L || K || J || H || G || F || D || S || A || Z || X || C || V || B || N || M)
      |                                                    ^
a.cc:5:57: error: 'O' was not declared in this scope
    5 |         if (a = Q || W || E || R || T || Y || U || I || O || P || L || K || J || H || G || F || D || S || A || Z || X || C || V || B || N || M)
      |                                                         ^
a.cc:5:62: error: 'P' was not declared in this scope
    5 |         if (a = Q || W || E || R || T || Y || U || I || O || P || L || K || J || H || G || F || D || S || A || Z || X || C || V || B || N || M)
      |                                                              ^
a.cc:5:67: error: 'L' was not declared in this scope
    5 |         if (a = Q || W || E || R || T || Y || U || I || O || P || L || K || J || H || G || F || D || S || A || Z || X || C || V || B || N || M)
      |                                                                   ^
a.cc:5:72: error: 'K' was not declared in this scope
    5 |         if (a = Q || W || E || R || T || Y || U || I || O || P || L || K || J || H || G || F || D || S || A || Z || X || C || V || B || N || M)
      |                                                                        ^
a.cc:5:77: error: 'J' was not declared in this scope
    5 |         if (a = Q || W || E || R || T || Y || U || I || O || P || L || K || J || H || G || F || D || S || A || Z || X || C || V || B || N || M)
      |                                                                             ^
a.cc:5:82: error: 'H' was not declared in this scope
    5 |         if (a = Q || W || E || R || T || Y || U || I || O || P || L || K || J || H || G || F || D || S || A || Z || X || C || V || B || N || M)
      |                                                                                  ^
a.cc:5:87: error: 'G' was not declared in this scope
    5 |         if (a = Q || W || E || R || T || Y || U || I || O || P || L || K || J || H || G || F || D || S || A || Z || X || C || V || B || N || M)
      |                                                                                       ^
a.cc:5:92: error: 'F' was not declared in this scope
    5 |         if (a = Q || W || E || R || T || Y || U || I || O || P || L || K || J || H || G || F || D || S || A || Z || X || C || V || B || N || M)
      |                                                                                            ^
a.cc:5:97: error: 'D' was not declared in this scope
    5 |         if (a = Q || W || E || R || T || Y || U || I || O || P || L || K || J || H || G || F || D || S || A || Z || X || C || V || B || N || M)
      |                                                                                                 ^
a.cc:5:102: error: 'S' was not declared in this scope
    5 |         if (a = Q || W || E || R || T || Y || U || I || O || P || L || K || J || H || G || F || D || S || A || Z || X || C || V || B || N || M)
      |                                                                                                      ^
a.cc:5:107: error: 'A' was not declared in this scope
    5 |         if (a = Q || W || E || R || T || Y || U || I || O || P || L || K || J || H || G || F || D || S || A || Z || X || C || V || B || N || M)
      |                                                                                                           ^
a.cc:5:112: error: 'Z' was not declared in this scope
    5 |         if (a = Q || W || E || R || T || Y || U || I || O || P || L || K || J || H || G || F || D || S || A || Z || X || C || V || B || N || M)
      |                                                                                                                ^
a.cc:5:117: error: 'X' was not declared in this scope
    5 |         if (a = Q || W || E || R || T || Y || U || I || O || P || L || K || J || H || G || F || D || S || A || Z || X || C || V || B || N || M)
      |                                                                                                                     ^
a.cc:5:122: error: 'C' was not declared in this scope
    5 |         if (a = Q || W || E || R || T || Y || U || I || O || P || L || K || J || H || G || F || D || S || A || Z || X || C || V || B || N || M)
      |                                                                                                                          ^
a.cc:5:127: error: 'V' was not declared in this scope
    5 |         if (a = Q || W || E || R || T || Y || U || I || O || P || L || K || J || H || G || F || D || S || A || Z || X || C || V || B || N || M)
      |                                                                                                                               ^
a.cc:5:132: error: 'B' was not declared in this scope
    5 |         if (a = Q || W || E || R || T || Y || U || I || O || P || L || K || J || H || G || F || D || S || A || Z || X || C || V || B || N || M)
      |                                                                                                                                    ^
a.cc:5:137: error: 'N' was not declared in this scope
    5 |         if (a = Q || W || E || R || T || Y || U || I || O || P || L || K || J || H || G || F || D || S || A || Z || X || C || V || B || N || M)
      |                                                                                                                                         ^
a.cc:5:142: error: 'M' was not declared in this scope
    5 |         if (a = Q || W || E || R || T || Y || U || I || O || P || L || K || J || H || G || F || D || S || A || Z || X || C || V || B || N || M)
      |                                                                                                                                              ^