Language: C++

#include<iostream>
#include<cstdio>
#include<cctype>
using namespace std;

int main(){
	char ch;
	while(1){
	scanf("%c",&ch);
	if(islower(ch)){cout << (char)toupper(ch);
	}else if (isupper(ch)){cout << (char)tolower(ch);
	}else if(ch == '\n'){break;
????????????}else[cout << ch;
	}
	cout << "\n";
	return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:13:1: error: expected primary-expression before '?' token
   13 | ????????????}else[cout << ch;
      | ^
a.cc:13:2: error: expected primary-expression before '?' token
   13 | ????????????}else[cout << ch;
      |  ^
a.cc:13:3: error: expected primary-expression before '?' token
   13 | ????????????}else[cout << ch;
      |   ^
a.cc:13:4: error: expected primary-expression before '?' token
   13 | ????????????}else[cout << ch;
      |    ^
a.cc:13:5: error: expected primary-expression before '?' token
   13 | ????????????}else[cout << ch;
      |     ^
a.cc:13:6: error: expected primary-expression before '?' token
   13 | ????????????}else[cout << ch;
      |      ^
a.cc:13:7: error: expected primary-expression before '?' token
   13 | ????????????}else[cout << ch;
      |       ^
a.cc:13:8: error: expected primary-expression before '?' token
   13 | ????????????}else[cout << ch;
      |        ^
a.cc:13:9: error: expected primary-expression before '?' token
   13 | ????????????}else[cout << ch;
      |         ^
a.cc:13:10: error: expected primary-expression before '?' token
   13 | ????????????}else[cout << ch;
      |          ^
a.cc:13:11: error: expected primary-expression before '?' token
   13 | ????????????}else[cout << ch;
      |           ^
a.cc:13:12: error: expected primary-expression before '?' token
   13 | ????????????}else[cout << ch;
      |            ^
a.cc:13:13: error: expected primary-expression before '}' token
   13 | ????????????}else[cout << ch;
      |             ^
a.cc:13:13: error: expected ':' before '}' token
   13 | ????????????}else[cout << ch;
      |             ^
      |             :
a.cc:13:13: error: expected primary-expression before '}' token
a.cc:13:13: error: expected ':' before '}' token
   13 | ????????????}else[cout << ch;
      |             ^
      |             :
a.cc:13:13: error: expected primary-expression before '}' token
a.cc:13:13: error: expected ':' before '}' token
   13 | ????????????}else[cout << ch;
      |             ^
      |             :
a.cc:13:13: error: expected primary-expression before '}' token
a.cc:13:13: error: expected ':' before '}' token
   13 | ????????????}else[cout << ch;
      |             ^
      |             :
a.cc:13:13: error: expected primary-expression before '}' token
a.cc:13:13: error: expected ':' before '}' token
   13 | ????????????}else[cout << ch;
      |             ^
      |             :
a.cc:13:13: error: expected primary-expression before '}' token
a.cc:13:13: error: expected ':' before '}' token
   13 | ????????????}else[cout << ch;
      |             ^
      |             :
a.cc:13:13: error: expected primary-expression before '}' token
a.cc:13:13: error: expected ':' before '}' token
   13 | ????????????}else[cout << ch;
      |             ^
      |             :
a.cc:13:13: error: expected primary-expression before '}' token
a.cc:13:13: error: expected ':' before '}' token
   13 | ????????????}else[cout << ch;
      |             ^
      |             :
a.cc:13:13: error: expected primary-expression before '}' token
a.cc:13:13: error: expected ':' before '}' token
   13 | ????????????}else[cout << ch;
      |             ^
      |             :
a.cc:13:13: error: expected primary-expression before '}' token
a.cc:13:13: error: expected ':' before '}' token
   13 | ????????????}else[cout << ch;
      |             ^
      |             :
a.cc:13:13: error: expected primary-expression before '}' token
a.cc:13:13: error: expected ':' before '}' token
   13 | ????????????}else[cout << ch;
      |             ^
      |             :
a.cc:13:13: error: expected primary-expression before '}' token
a.cc:13:13: error: expected ':' before '}' token
   13 | ????????????}else[cout << ch;
      |             ^
      |             :
a.cc:13:13: error: expected primary-expression before '}' token
a.cc:13:19: warning: capture of variable 'std::cout' with non-automatic storage duration
   13 | ????????????}else[cout << ch;
      |                   ^~~~
In file included from a.cc:1:
/usr/include/c++/14/iostream:63:18: note: 'std::ostream std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:13:23: error: expected ',' before '<<' token
   13 | ????????????}else[cout << ch;
      |                       ^~~
      |                       ,
a.cc:13:24: error: expected identifier before '<<' token
   13 | ????????????}else[cout << ch;
      |                        ^~
a.cc:13:29: error: expected ']' before ';' token
   13 | ????????????}else[cout << ch;
      |                             ^
      |                             ]
a.cc: In lambda function:
a.cc:13:29: error: expected '{' before ';' token