Language: C++

#include <iostream>
#include <string>
#include <stdlib.h> 
using namespace std;

int main() {
	string s;
	getline(cin,s);
	for(int i=0;i<=s.size();i++){
		if(isalpha(s[i]))
			cout<toupper(s[i]);
		else cout<<s[i];
	}cout<<endl;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:11:29: error: no match for 'operator<' (operand types are 'std::ostream' {aka 'std::basic_ostream<char>'} and 'int')
   11 |                         cout<toupper(s[i]);
      |                         ~~~~^~~~~~~~~~~~~~
      |                         |           |
      |                         |           int
      |                         std::ostream {aka std::basic_ostream<char>}
a.cc:11:29: note: candidate: 'operator<(int, int)' (built-in)
   11 |                         cout<toupper(s[i]);
      |                         ~~~~^~~~~~~~~~~~~~
a.cc:11:29: note:   no known conversion for argument 1 from 'std::ostream' {aka 'std::basic_ostream<char>'} to 'int'
In file included from /usr/include/c++/14/string:48,
                 from /usr/include/c++/14/bits/locale_classes.h:40,
                 from /usr/include/c++/14/bits/ios_base.h:41,
                 from /usr/include/c++/14/ios:44,
                 from /usr/include/c++/14/ostream:40,
                 from /usr/include/c++/14/iostream:41,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_iterator.h:448:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator<(const reverse_iterator<_Iterator>&, const reverse_iterator<_Iterator>&)'
  448 |     operator<(const reverse_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:448:5: note:   template argument deduction/substitution failed:
a.cc:11:42: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'const std::reverse_iterator<_Iterator>'
   11 |                         cout<toupper(s[i]);
      |                                          ^
/usr/include/c++/14/bits/stl_iterator.h:493:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator<(const reverse_iterator<_Iterator>&, const reverse_iterator<_IteratorR>&)'
  493 |     operator<(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:493:5: note:   template argument deduction/substitution failed:
a.cc:11:42: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'const std::reverse_iterator<_Iterator>'
   11 |                         cout<toupper(s[i]);
      |                                          ^
/usr/include/c++/14/bits/stl_iterator.h:1694:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator<(const move_iterator<_IteratorL>&, const move_iterator<_IteratorR>&)'
 1694 |     operator<(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1694:5: note:   template argument deduction/substitution failed:
a.cc:11:42: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'const std::move_iterator<_IteratorL>'
   11 |                         cout<toupper(s[i]);
      |                                          ^
/usr/include/c++/14/bits/stl_iterator.h:1760:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator<(const move_iterator<_IteratorL>&, const move_iterator<_IteratorL>&)'
 1760 |     operator<(const move_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1760:5: note:   template argument deduction/substitution failed:
a.cc:11:42: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'const std::move_iterator<_IteratorL>'
   11 |                         cout<toupper(s[i]);
      |                                          ^
In file included from /usr/include/c++/14/bits/stl_algobase.h:64,
                 from /usr/include/c++/14/string:51:
/usr/include/c++/14/bits/stl_pair.h:1045:5: note: candidate: 'template<class _T1, class _T2> constexpr bool std::operator<(const pair<_T1, _T2>&, const pair<_T1, _T2>&)'
 1045 |     operator<(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_pair.h:1045:5: note:   template argument deduction/substitution failed:
a.cc:11:42: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'const std::pair<_T1, _T2>'
   11 |                         cout<toupper(s[i]);
      |                                          ^
In file included from /usr/include/c++/14/bits/basic_string.h:47,
                 from /usr/include/c++/14/string:54:
/usr/include/c++/14/string_view:673:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator<(basic_string_view<_CharT, _Traits>, basic_string_view<_CharT, _Traits>)'
  673 |     operator< (basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:673:5: note:   template argument deduction/substitution failed:
a.cc:11:42: note:   'std::basic_ostream<char>' is not derived from 'std::basic_string_view<_CharT, _Traits>'
   11 |                         cout<toupper(s[i]);
      |                                          ^
/usr/include/c++/14/string_view:680:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator<(basic_string_view<_CharT, _Traits>, __type_identity_t<basic_string_view<_CharT, _Traits> >)'
  680 |     operator< (basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:680:5: note:   template argument deduction/substitution failed:
a.cc:11:42: note:   'std::basic_ostream<char>' is not derived from 'std::basic_string_view<_CharT, _Traits>'
   11 |                         cout<toupper(s[i]);
      |                                          ^
/usr/include/c++/14/string_view:688:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator<(__type_identity_t<basic_string_view<_CharT, _Traits> >, basic_string_view<_CharT, _Traits>)'
  688 |     operator< (__type_identity_t<basic_string_view<_CharT, _Traits>> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:688:5: note:   template argument deduction/substitution failed:
a.cc:11:42: note:   mismatched types 'std::basic_string_view<_CharT, _Traits>' and 'int'
   11 |                         cout<toupper(s[i]);
      |                                          ^
/usr/include/c++/14/bits/basic_string.h:3874:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3874 |     operator<(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3874:5: note:   template argument deduction/substitution failed:
a.cc:11:42: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   11 |                         cout<toupper(s[i]);
      |                                          ^
/usr/include/c++/14/bits/basic_string.h:3888:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const _CharT*)'
 3888 |     operator<(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3888:5: note:   template argument deduction/substitution failed:
a.cc:11:42: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   11 |                         cout<toupper(s[i]);
      |                                          ^
/usr/include/c++/14/bits/basic_string.h:3901:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const _CharT*, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3901 |     operator<(const _CharT* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3901:5: note:   template argument deduction/substitution failed:
a.cc:11:42: note:   mismatched types 'const _CharT*' and 'std::basic_ostream<char>'
   11 |                         cout<toupper(s[i]);
      |                                          ^
In file included from /usr/include/c++/14/bits/memory_resource.h:47,
                 from /usr/include/c++/14/string:68:
/usr/include/c++/14/tuple:2600:5: note: candidate: 'template<class ... _TElements, class ... _UElements> constexpr bool std::operator<(const tuple<_UTypes ...>&, const tuple<_Elements ...>&)'
 2600 |     operator<(const tuple<_TElements...>& __t,
      |     ^~~~~~~~
/usr/include/c++/14/tuple:2600:5: note:   template argument deduction/substitution failed:
a.cc:11:42: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'const std::tuple<_UTypes ...>'
   11 |                         cout<toupper(s[i]);
      |                                          ^
In file included from /usr/include/c++/14/bits/ios_base.h:46:
/usr/include/c++/14/system_error:324:3: note: candidate: 'bool std::operator<(const error_code&, const error_code&)'
  324 |   operator<(const error_code& __lhs, const error_code& __rhs) noexcept
      |   ^~~~~~~~
/usr/include/c++/14/system_error:324:31: note:   no known conversion for argument 1 from 'std::ostream' {aka 'std::basic_ostream<char>'} to 'const std::error_code&'
  324 |   operator<(const error_code& __lhs, const error_code& __rhs) noexcept
      |             ~~~~~~~~~~~~~~~~~~^~~~~
/usr/include/c++/14/system_error:507:3: note: candidate: 'bool std::operator<(const error_condition&, const error_condition&)'
  507 |   operator<(const error_condition& __lhs,
      |   ^~~~~~~~
/usr/include/c++/14/system_error:507:36: note:   no known conversion for argument 1 from 'std::ostream' {aka 'std::basic_ostream<char>'} to 'const std::error_condition&'
  507 |   operator<(const error_condition& __lhs,
      |             ~~~~~~~~~~~~~~~~~~~~~~~^~~~~