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#include <iostream>
using namespace std;
 
template<size_t>
class intWrapper
{
    int value;
public:
    explicit intWrapper(int value = 0) : value(value) {}
    explicit operator int() const { return value; }
    //...
 
    // the same error occurs with and without this operator defined!
    bool operator==(const intWrapper &rhs) const { return value == rhs.value; }
};
 
using BookID = intWrapper<1>;
using CustomerID = intWrapper<2>;
 
int main()
{
	BookID bookId;
	CustomerID custId;
 
	//bookId = custId; // <-- fails!
 
	if (bookId == custId) {} // <-- fails!
 
	return 0;
}

prog.cpp: In function ‘int main()’:
prog.cpp:27:13: error: no match for ‘operator==’ (operand types are ‘BookID’ {aka ‘intWrapper<1>’} and ‘CustomerID’ {aka ‘intWrapper<2>’})
  if (bookId == custId) {} // <-- fails!
      ~~~~~~~^~~~~~~~~
prog.cpp:27:13: note: candidate: ‘operator==(int, int)’ <built-in>
prog.cpp:27:13: note:   no known conversion for argument 2 from ‘CustomerID’ {aka ‘intWrapper<2>’} to ‘int’
prog.cpp:14:10: note: candidate: ‘bool intWrapper<<anonymous> >::operator==(const intWrapper<<anonymous> >&) const [with long unsigned int <anonymous> = 1]’
     bool operator==(const intWrapper &rhs) const { return value == rhs.value; }
          ^~~~~~~~
prog.cpp:14:10: note:   no known conversion for argument 1 from ‘CustomerID’ {aka ‘intWrapper<2>’} to ‘const intWrapper<1>&’
In file included from /usr/include/c++/8/iosfwd:40,
                 from /usr/include/c++/8/ios:38,
                 from /usr/include/c++/8/ostream:38,
                 from /usr/include/c++/8/iostream:39,
                 from prog.cpp:1:
/usr/include/c++/8/bits/postypes.h:216:5: note: candidate: ‘template<class _StateT> bool std::operator==(const std::fpos<_StateT>&, const std::fpos<_StateT>&)’
     operator==(const fpos<_StateT>& __lhs, const fpos<_StateT>& __rhs)
     ^~~~~~~~
/usr/include/c++/8/bits/postypes.h:216:5: note:   template argument deduction/substitution failed:
prog.cpp:27:16: note:   ‘BookID’ {aka ‘intWrapper<1>’} is not derived from ‘const std::fpos<_StateT>’
  if (bookId == custId) {} // <-- fails!
                ^~~~~~
In file included from /usr/include/c++/8/bits/stl_algobase.h:64,
                 from /usr/include/c++/8/bits/char_traits.h:39,
                 from /usr/include/c++/8/ios:40,
                 from /usr/include/c++/8/ostream:38,
                 from /usr/include/c++/8/iostream:39,
                 from prog.cpp:1:
/usr/include/c++/8/bits/stl_pair.h:448:5: note: candidate: ‘template<class _T1, class _T2> constexpr bool std::operator==(const std::pair<_T1, _T2>&, const std::pair<_T1, _T2>&)’
     operator==(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
     ^~~~~~~~
/usr/include/c++/8/bits/stl_pair.h:448:5: note:   template argument deduction/substitution failed:
prog.cpp:27:16: note:   ‘BookID’ {aka ‘intWrapper<1>’} is not derived from ‘const std::pair<_T1, _T2>’
  if (bookId == custId) {} // <-- fails!
                ^~~~~~
In file included from /usr/include/c++/8/bits/stl_algobase.h:67,
                 from /usr/include/c++/8/bits/char_traits.h:39,
                 from /usr/include/c++/8/ios:40,
                 from /usr/include/c++/8/ostream:38,
                 from /usr/include/c++/8/iostream:39,
                 from prog.cpp:1:
/usr/include/c++/8/bits/stl_iterator.h:302:5: note: candidate: ‘template<class _Iterator> bool std::operator==(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_Iterator>&)’
     operator==(const reverse_iterator<_Iterator>& __x,
     ^~~~~~~~
/usr/include/c++/8/bits/stl_iterator.h:302:5: note:   template argument deduction/substitution failed:
prog.cpp:27:16: note:   ‘BookID’ {aka ‘intWrapper<1>’} is not derived from ‘const std::reverse_iterator<_Iterator>’
  if (bookId == custId) {} // <-- fails!
                ^~~~~~
In file included from /usr/include/c++/8/bits/stl_algobase.h:67,
                 from /usr/include/c++/8/bits/char_traits.h:39,
                 from /usr/include/c++/8/ios:40,
                 from /usr/include/c++/8/ostream:38,
                 from /usr/include/c++/8/iostream:39,
                 from prog.cpp:1:
/usr/include/c++/8/bits/stl_iterator.h:340:5: note: candidate: ‘template<class _IteratorL, class _IteratorR> bool std::operator==(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_IteratorR>&)’
     operator==(const reverse_iterator<_IteratorL>& __x,
     ^~~~~~~~
/usr/include/c++/8/bits/stl_iterator.h:340:5: note:   template argument deduction/substitution failed:
prog.cpp:27:16: note:   ‘BookID’ {aka ‘intWrapper<1>’} is not derived from ‘const std::reverse_iterator<_Iterator>’
  if (bookId == custId) {} // <-- fails!
                ^~~~~~
In file included from /usr/include/c++/8/bits/stl_algobase.h:67,
                 from /usr/include/c++/8/bits/char_traits.h:39,
                 from /usr/include/c++/8/ios:40,
                 from /usr/include/c++/8/ostream:38,
                 from /usr/include/c++/8/iostream:39,
                 from prog.cpp:1:
/usr/include/c++/8/bits/stl_iterator.h:1115:5: note: candidate: ‘template<class _IteratorL, class _IteratorR> bool std::operator==(const std::move_iterator<_IteratorL>&, const std::move_iterator<_IteratorR>&)’
     operator==(const move_iterator<_IteratorL>& __x,
     ^~~~~~~~
/usr/include/c++/8/bits/stl_iterator.h:1115:5: note:   template argument deduction/substitution failed:
prog.cpp:27:16: note:   ‘BookID’ {aka ‘intWrapper<1>’} is not derived from ‘const std::move_iterator<_IteratorL>’
  if (bookId == custId) {} // <-- fails!
                ^~~~~~
In file included from /usr/include/c++/8/bits/stl_algobase.h:67,
                 from /usr/include/c++/8/bits/char_traits.h:39,
                 from /usr/include/c++/8/ios:40,
                 from /usr/include/c++/8/ostream:38,
                 from /usr/include/c++/8/iostream:39,
                 from prog.cpp:1:
/usr/include/c++/8/bits/stl_iterator.h:1121:5: note: candidate: ‘template<class _Iterator> bool std::operator==(const std::move_iterator<_IteratorL>&, const std::move_iterator<_IteratorL>&)’
     operator==(const move_iterator<_Iterator>& __x,
     ^~~~~~~~
/usr/include/c++/8/bits/stl_iterator.h:1121:5: note:   template argument deduction/substitution failed:
prog.cpp:27:16: note:   ‘BookID’ {aka ‘intWrapper<1>’} is not derived from ‘const std::move_iterator<_IteratorL>’
  if (bookId == custId) {} // <-- fails!
                ^~~~~~
In file included from /usr/include/c++/8/string:41,
                 from /usr/include/c++/8/bits/locale_classes.h:40,
                 from /usr/include/c++/8/bits/ios_base.h:41,
                 from /usr/include/c++/8/ios:42,
                 from /usr/include/c++/8/ostream:38,
                 from /usr/include/c++/8/iostream:39,
                 from prog.cpp:1:
/usr/include/c++/8/bits/allocator.h:146:5: note: candidate: ‘template<class _T1, class _T2> bool std::operator==(const std::allocator<_CharT>&, const std::allocator<_T2>&)’
     operator==(const allocator<_T1>&, const allocator<_T2>&)
     ^~~~~~~~
/usr/include/c++/8/bits/allocator.h:146:5: note:   template argument deduction/substitution failed:
prog.cpp:27:16: note:   ‘BookID’ {aka ‘intWrapper<1>’} is not derived from ‘const std::allocator<_CharT>’
  if (bookId == custId) {} // <-- fails!
                ^~~~~~
In file included from /usr/include/c++/8/string:41,
                 from /usr/include/c++/8/bits/locale_classes.h:40,
                 from /usr/include/c++/8/bits/ios_base.h:41,
                 from /usr/include/c++/8/ios:42,
                 from /usr/include/c++/8/ostream:38,
                 from /usr/include/c++/8/iostream:39,
                 from prog.cpp:1:
/usr/include/c++/8/bits/allocator.h:152:5: note: candidate: ‘template<class _Tp> bool std::operator==(const std::allocator<_CharT>&, const std::allocator<_CharT>&)’
     operator==(const allocator<_Tp>&, const allocator<_Tp>&)
     ^~~~~~~~
/usr/include/c++/8/bits/allocator.h:152:5: note:   template argument deduction/substitution failed:
prog.cpp:27:16: note:   ‘BookID’ {aka ‘intWrapper<1>’} is not derived from ‘const std::allocator<_CharT>’
  if (bookId == custId) {} // <-- fails!
                ^~~~~~
In file included from /usr/include/c++/8/string:52,
                 from /usr/include/c++/8/bits/locale_classes.h:40,
                 from /usr/include/c++/8/bits/ios_base.h:41,
                 from /usr/include/c++/8/ios:42,
                 from /usr/include/c++/8/ostream:38,
                 from /usr/include/c++/8/iostream:39,
                 from prog.cpp:1:
/usr/include/c++/8/bits/basic_string.h:6048:5: note: candidate: ‘template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&)’
     operator==(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
     ^~~~~~~~
/usr/include/c++/8/bits/basic_string.h:6048:5: note:   template argument deduction/substitution failed:
prog.cpp:27:16: note:   ‘BookID’ {aka ‘intWrapper<1>’} is not derived from ‘const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>’
  if (bookId == custId) {} // <-- fails!
                ^~~~~~
In file included from /usr/include/c++/8/string:52,
                 from /usr/include/c++/8/bits/locale_classes.h:40,
                 from /usr/include/c++/8/bits/ios_base.h:41,
                 from /usr/include/c++/8/ios:42,
                 from /usr/include/c++/8/ostream:38,
                 from /usr/include/c++/8/iostream:39,
                 from prog.cpp:1:
/usr/include/c++/8/bits/basic_string.h:6056:5: note: candidate: ‘template<class _CharT> typename __gnu_cxx::__enable_if<std::__is_char<_Tp>::__value, bool>::__type std::operator==(const std::__cxx11::basic_string<_CharT>&, const std::__cxx11::basic_string<_CharT>&)’
     operator==(const basic_string<_CharT>& __lhs,
     ^~~~~~~~
/usr/include/c++/8/bits/basic_string.h:6056:5: note:   template argument deduction/substitution failed:
prog.cpp:27:16: note:   ‘BookID’ {aka ‘intWrapper<1>’} is not derived from ‘const std::__cxx11::basic_string<_CharT>’
  if (bookId == custId) {} // <-- fails!
                ^~~~~~
In file included from /usr/include/c++/8/string:52,
                 from /usr/include/c++/8/bits/locale_classes.h:40,
                 from /usr/include/c++/8/bits/ios_base.h:41,
                 from /usr/include/c++/8/ios:42,
                 from /usr/include/c++/8/ostream:38,
                 from /usr/include/c++/8/iostream:39,
                 from prog.cpp:1:
/usr/include/c++/8/bits/basic_string.h:6070:5: note: candidate: ‘template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const _CharT*, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&)’
     operator==(const _CharT* __lhs,
     ^~~~~~~~
/usr/include/c++/8/bits/basic_string.h:6070:5: note:   template argument deduction/substitution failed:
prog.cpp:27:16: note:   mismatched types ‘const _CharT*’ and ‘intWrapper<1>’
  if (bookId == custId) {} // <-- fails!
                ^~~~~~
In file included from /usr/include/c++/8/string:52,
                 from /usr/include/c++/8/bits/locale_classes.h:40,
                 from /usr/include/c++/8/bits/ios_base.h:41,
                 from /usr/include/c++/8/ios:42,
                 from /usr/include/c++/8/ostream:38,
                 from /usr/include/c++/8/iostream:39,
                 from prog.cpp:1:
/usr/include/c++/8/bits/basic_string.h:6082:5: note: candidate: ‘template<class _CharT, class _Traits, class _Alloc> bool std::operator==(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, const _CharT*)’
     operator==(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
     ^~~~~~~~
/usr/include/c++/8/bits/basic_string.h:6082:5: note:   template argument deduction/substitution failed:
prog.cpp:27:16: note:   ‘BookID’ {aka ‘intWrapper<1>’} is not derived from ‘const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>’
  if (bookId == custId) {} // <-- fails!
                ^~~~~~
In file included from /usr/include/c++/8/bits/ios_base.h:46,
                 from /usr/include/c++/8/ios:42,
                 from /usr/include/c++/8/ostream:38,
                 from /usr/include/c++/8/iostream:39,
                 from prog.cpp:1:
/usr/include/c++/8/system_error:292:3: note: candidate: ‘bool std::operator==(const std::error_code&, const std::error_code&)’
   operator==(const error_code& __lhs, const error_code& __rhs) noexcept
   ^~~~~~~~
/usr/include/c++/8/system_error:292:3: note:   no known conversion for argument 1 from ‘BookID’ {aka ‘intWrapper<1>’} to ‘const std::error_code&’
/usr/include/c++/8/system_error:297:3: note: candidate: ‘bool std::operator==(const std::error_code&, const std::error_condition&)’
   operator==(const error_code& __lhs, const error_condition& __rhs) noexcept
   ^~~~~~~~
/usr/include/c++/8/system_error:297:3: note:   no known conversion for argument 1 from ‘BookID’ {aka ‘intWrapper<1>’} to ‘const std::error_code&’
/usr/include/c++/8/system_error:304:3: note: candidate: ‘bool std::operator==(const std::error_condition&, const std::error_code&)’
   operator==(const error_condition& __lhs, const error_code& __rhs) noexcept
   ^~~~~~~~
/usr/include/c++/8/system_error:304:3: note:   no known conversion for argument 1 from ‘BookID’ {aka ‘intWrapper<1>’} to ‘const std::error_condition&’
/usr/include/c++/8/system_error:311:3: note: candidate: ‘bool std::operator==(const std::error_condition&, const std::error_condition&)’
   operator==(const error_condition& __lhs,
   ^~~~~~~~
/usr/include/c++/8/system_error:311:3: note:   no known conversion for argument 1 from ‘BookID’ {aka ‘intWrapper<1>’} to ‘const std::error_condition&’
In file included from /usr/include/c++/8/bits/locale_facets.h:48,
                 from /usr/include/c++/8/bits/basic_ios.h:37,
                 from /usr/include/c++/8/ios:44,
                 from /usr/include/c++/8/ostream:38,
                 from /usr/include/c++/8/iostream:39,
                 from prog.cpp:1:
/usr/include/c++/8/bits/streambuf_iterator.h:203:5: note: candidate: ‘template<class _CharT, class _Traits> bool std::operator==(const std::istreambuf_iterator<_CharT, _Traits>&, const std::istreambuf_iterator<_CharT, _Traits>&)’
     operator==(const istreambuf_iterator<_CharT, _Traits>& __a,
     ^~~~~~~~
/usr/include/c++/8/bits/streambuf_iterator.h:203:5: note:   template argument deduction/substitution failed:
prog.cpp:27:16: note:   ‘BookID’ {aka ‘intWrapper<1>’} is not derived from ‘const std::istreambuf_iterator<_CharT, _Traits>’
  if (bookId == custId) {} // <-- fails!
                ^~~~~~

Standard output is empty