Language: C++

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

int main(){
  string S;
  cin >> S;
  
  if(S = 'a' || S = 'i' || S = 'u' || S = 'e' || S = 'o'){
    cout << "vowel" << endl;
  }
  else{
	cout << "consonant" << endl;
  }
}

Compiler output:
a.cc: In function 'int main()':
a.cc:8:14: error: no match for 'operator||' (operand types are 'char' and 'std::string' {aka 'std::__cxx11::basic_string<char>'})
    8 |   if(S = 'a' || S = 'i' || S = 'u' || S = 'e' || S = 'o'){
      |          ~~~ ^~ ~
      |          |      |
      |          char   std::string {aka std::__cxx11::basic_string<char>}
a.cc:8:14: note: candidate: 'operator||(bool, bool)' (built-in)
    8 |   if(S = 'a' || S = 'i' || S = 'u' || S = 'e' || S = 'o'){
      |          ~~~~^~~~
a.cc:8:14: note:   no known conversion for argument 2 from 'std::string' {aka 'std::__cxx11::basic_string<char>'} to 'bool'
In file included from /usr/include/c++/14/valarray:605,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:166,
                 from a.cc:1:
/usr/include/c++/14/bits/valarray_after.h:416:5: note: candidate: 'template<class _Dom1, class _Dom2> std::_Expr<std::__detail::_BinClos<std::__logical_or, std::_Expr, std::_Expr, _Dom1, _Dom2>, typename std::__fun<std::__logical_or, typename _Dom1::value_type>::result_type> std::operator||(const _Expr<_Dom1, typename _Dom1::value_type>&, const _Expr<_Dom2, typename _Dom2::value_type>&)'
  416 |     _DEFINE_EXPR_BINARY_OPERATOR(||, struct std::__logical_or)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:416:5: note:   template argument deduction/substitution failed:
a.cc:8:17: note:   mismatched types 'const std::_Expr<_Dom1, typename _Dom1::value_type>' and 'char'
    8 |   if(S = 'a' || S = 'i' || S = 'u' || S = 'e' || S = 'o'){
      |                 ^
/usr/include/c++/14/bits/valarray_after.h:416:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__logical_or, std::_Expr, std::_Constant, _Dom, typename _Dom::value_type>, typename std::__fun<std::__logical_or, typename _Dom1::value_type>::result_type> std::operator||(const _Expr<_Dom1, typename _Dom1::value_type>&, const typename _Dom::value_type&)'
  416 |     _DEFINE_EXPR_BINARY_OPERATOR(||, struct std::__logical_or)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:416:5: note:   template argument deduction/substitution failed:
a.cc:8:17: note:   mismatched types 'const std::_Expr<_Dom1, typename _Dom1::value_type>' and 'char'
    8 |   if(S = 'a' || S = 'i' || S = 'u' || S = 'e' || S = 'o'){
      |                 ^
/usr/include/c++/14/bits/valarray_after.h:416:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__logical_or, std::_Constant, std::_Expr, typename _Dom::value_type, _Dom>, typename std::__fun<std::__logical_or, typename _Dom1::value_type>::result_type> std::operator||(const typename _Dom::value_type&, const _Expr<_Dom1, typename _Dom1::value_type>&)'
  416 |     _DEFINE_EXPR_BINARY_OPERATOR(||, struct std::__logical_or)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:416:5: note:   template argument deduction/substitution failed:
a.cc:8:17: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
    8 |   if(S = 'a' || S = 'i' || S = 'u' || S = 'e' || S = 'o'){
      |                 ^
/usr/include/c++/14/bits/valarray_after.h:416:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__logical_or, std::_Expr, std::_ValArray, _Dom, typename _Dom::value_type>, typename std::__fun<std::__logical_or, typename _Dom1::value_type>::result_type> std::operator||(const _Expr<_Dom1, typename _Dom1::value_type>&, const valarray<typename _Dom::value_type>&)'
  416 |     _DEFINE_EXPR_BINARY_OPERATOR(||, struct std::__logical_or)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:416:5: note:   template argument deduction/substitution failed:
a.cc:8:17: note:   mismatched types 'const std::_Expr<_Dom1, typename _Dom1::value_type>' and 'char'
    8 |   if(S = 'a' || S = 'i' || S = 'u' || S = 'e' || S = 'o'){
      |                 ^
/usr/include/c++/14/bits/valarray_after.h:416:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__logical_or, std::_ValArray, std::_Expr, typename _Dom::value_type, _Dom>, typename std::__fun<std::__logical_or, typename _Dom1::value_type>::result_type> std::operator||(const valarray<typename _Dom::value_type>&, const _Expr<_Dom1, typename _Dom1::value_type>&)'
  416 |     _DEFINE_EXPR_BINARY_OPERATOR(||, struct std::__logical_or)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:416:5: note:   template argument deduction/substitution failed:
a.cc:8:17: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
    8 |   if(S = 'a' || S = 'i' || S = 'u' || S = 'e' || S = 'o'){
      |                 ^
/usr/include/c++/14/valarray:1207:1: note: candidate: 'template<class _Tp> std::_Expr<std::__detail::_BinClos<std::__logical_or, std::_ValArray, std::_ValArray, _Tp, _Tp>, typename std::__fun<std::__logical_or, _Tp>::result_type> std::operator||(const valarray<_Tp>&, const valarray<_Tp>&)'
 1207 | _DEFINE_BINARY_OPERATOR(||, __logical_or)
      | ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/valarray:1207:1: note:   template argument deduction/substitution failed:
a.cc:8:17: note:   mismatched types 'const std::valarray<_Tp>' and 'char'
    8 |   if(S = 'a' || S = 'i' || S = 'u' || S = 'e' || S = 'o'){
      |                 ^
/usr/include/c++/14/valarray:1207:1: note: candidate: 'template<class _Tp> std::_Expr<std::__detail::_BinClos<std::__logical_or, std::_ValArray, std::_Constant, _Tp, _Tp>, typename std::__fun<std::__logical_or, _Tp>::result_type> std::operator||(const valarray<_Tp>&, const typename valarray<_Tp>::value_type&)'
 1207 | _DEFINE_BINARY_OPERATOR(||, __logical_or)
      | ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/valarray:1207:1: note:   template argument deduction/substitution failed:
a.cc:8:17: note:   mismatched types 'const std::valarray<_Tp>' and 'char'
    8 |   if(S = 'a' || S = 'i' || S = 'u' || S = 'e' || S = 'o'){
      |                 ^
/usr/include/c++/14/valarray:1207:1: note: candidate: 'template<class _Tp> std::_Expr<std::__detail::_BinClos<std::__logical_or, std::_Constant, std::_ValArray, _Tp, _Tp>, typename std::__fun<std::__logical_or, _Tp>::result_type> std::operator||(const typename valarray<_Tp>::value_type&, const valarray<_Tp>&)'
 1207 | _DEFINE_BINARY_OPERATOR(||, __logical_or)
      | ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/valarray:1207:1: note:   template argument deduction/substitution failed:
a.cc:8:17: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::valarray<_Tp>'
    8 |   if(S = 'a' || S = 'i' || S = 'u' || S = 'e' || S = 'o'){
      |                 ^
a.cc:8:25: error: no match for 'operator||' (operand types are 'char' and 'std::string' {aka 'std::__cxx11::basic_string<char>'})
    8 |   if(S = 'a' || S = 'i' || S = 'u' || S = 'e' || S = 'o'){
      |                     ~~~ ^~ ~
      |                     |      |
      |                     char   std::string {aka std::__cxx11::basic_string<char>}
a.cc:8:25: note: candidate: 'operator||(bool, bool)' (built-in)
    8 |   if(S = 'a' || S = 'i' || S = 'u' || S = 'e' || S = 'o'){
      |                     ~~~~^~~~
a.cc:8:25: note:   no known conversion for argument 2 from 'std::string' {aka 'std::__cxx11::basic_string<char>'} to 'bool'
/usr/include/c++/14/bits/valarray_after.h:416:5: note: candidate: 'template<class _Dom1, class _Dom2> std::_Expr<std::__detail::_BinClos<std::__logical_or, std::_Expr, std::_Expr, _Dom1, _Dom2>, typename std::__fun<std::__logical_or, typename _Dom1::value_type>::result_type> std::operator||(const _Expr<_Dom1, typename _Dom1::value_type>&, const _Expr<_Dom2, typename _Dom2::value_type>&)'
  416 |     _DEFINE_EXPR_BINARY_OPERATOR(||, struct std::__logical_or)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:416:5: note:   template argument deduction/substitution failed:
a.cc:8:28: note:   mismatched types 'const std::_Expr<_Dom1, typename _Dom1::value_type>' and 'char'
    8 |   if(S = 'a' || S = 'i' || S = 'u' || S = 'e' || S = 'o'){
      |                            ^
/usr/include/c++/14/bits/valarray_after.h:416:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__logical_or, std::_Expr, std::_Constant, _Dom, typename _Dom::value_type>, typename std::__fun<std::__logical_or, typename _Dom1::value_type>::result_type> std::operator||(const _Expr<_Dom1, typename _Dom1::value_type>&, const typename _Dom::value_type&)'
  416 |     _DEFINE_EXPR_BINARY_OPERATOR(||, struct std::__logical_or)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:416:5: note:   template argument deduction/substitution failed:
a.cc:8:28: note:   mismatched types 'const std::_Expr<_Dom1, typename _Dom1::value_type>' and 'char'
    8 |   if(S = 'a' || S = 'i' || S = 'u' || S = 'e' || S = 'o'){
      |                            ^
/usr/include/c++/14/bits/valarray_after.h:416:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__logical_or, std::_Constant, std::_Expr, typename _Dom::value_type, _Dom>, typename std::__fun<std::__logical_or, typename _Dom1::value_type>::result_type> std::operator||(const typename _Dom::value_type&, const _Expr<_Dom1, typename _Dom1::value_type>&)'
  416 |     _DEFINE_EXPR_BINARY_OPERATOR(||, struct std::__logical_or)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:416:5: note:   template argument deduction/substitution failed:
a.cc:8:28: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
    8 |   if(S = 'a' || S = 'i' || S = 'u' || S = 'e' || S = 'o'){
      |                            ^
/usr/include/c++/14/bits/valarray_after.h:416:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__logical_or, std::_Expr, std::_ValArray, _Dom, typename _Dom::value_type>, typename std::