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#include <iostream>
using namespace std;
 
int main() {
	auto foo = +[](int a) { std::cout << "foo " << a << std::endl; };
	auto bar = +[](auto a) { std::cout << "bar " << a << std::endl; };
 
	foo(5);
	bar(6);
	// your code goes here
	return 0;
}

prog.cpp: In function ‘int main()’:
prog.cpp:6:13: error: no match for ‘operator+’ (operand type is ‘main()::<lambda(auto:1)>’)
  auto bar = +[](auto a) { std::cout << "bar " << a << std::endl; };
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:400:5: note: candidate: ‘template<class _Iterator> std::reverse_iterator<_Iterator> std::operator+(typename std::reverse_iterator<_Iterator>::difference_type, const std::reverse_iterator<_Iterator>&)’
     operator+(typename reverse_iterator<_Iterator>::difference_type __n,
     ^~~~~~~~
/usr/include/c++/8/bits/stl_iterator.h:400:5: note:   template argument deduction/substitution failed:
prog.cpp:6:66: note:   candidate expects 2 arguments, 1 provided
  auto bar = +[](auto a) { std::cout << "bar " << a << std::endl; };
                                                                  ^
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:1195:5: note: candidate: ‘template<class _Iterator> std::move_iterator<_IteratorL> std::operator+(typename std::move_iterator<_IteratorL>::difference_type, const std::move_iterator<_IteratorL>&)’
     operator+(typename move_iterator<_Iterator>::difference_type __n,
     ^~~~~~~~
/usr/include/c++/8/bits/stl_iterator.h:1195:5: note:   template argument deduction/substitution failed:
prog.cpp:6:66: note:   candidate expects 2 arguments, 1 provided
  auto bar = +[](auto a) { std::cout << "bar " << a << std::endl; };
                                                                  ^
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:5927:5: note: candidate: ‘template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Alloc> 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:5927:5: note:   template argument deduction/substitution failed:
prog.cpp:6:66: note:   ‘main()::<lambda(auto:1)>’ is not derived from ‘const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>’
  auto bar = +[](auto a) { std::cout << "bar " << a << std::endl; };
                                                                  ^
In file included from /usr/include/c++/8/string:53,
                 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.tcc:1157:5: note: candidate: ‘template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Alloc> std::operator+(const _CharT*, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&)’
     operator+(const _CharT* __lhs,
     ^~~~~~~~
/usr/include/c++/8/bits/basic_string.tcc:1157:5: note:   template argument deduction/substitution failed:
prog.cpp:6:66: note:   mismatched types ‘const _CharT*’ and ‘main()::<lambda(auto:1)>’
  auto bar = +[](auto a) { std::cout << "bar " << a << std::endl; };
                                                                  ^
In file included from /usr/include/c++/8/string:53,
                 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.tcc:1173:5: note: candidate: ‘template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Alloc> std::operator+(_CharT, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&)’
     operator+(_CharT __lhs, const basic_string<_CharT, _Traits, _Alloc>& __rhs)
     ^~~~~~~~
/usr/include/c++/8/bits/basic_string.tcc:1173:5: note:   template argument deduction/substitution failed:
prog.cpp:6:66: note:   candidate expects 2 arguments, 1 provided
  auto bar = +[](auto a) { std::cout << "bar " << a << std::endl; };
                                                                  ^
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:5964:5: note: candidate: ‘template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Alloc> 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:5964:5: note:   template argument deduction/substitution failed:
prog.cpp:6:66: note:   ‘main()::<lambda(auto:1)>’ is not derived from ‘const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>’
  auto bar = +[](auto a) { std::cout << "bar " << a << std::endl; };
                                                                  ^
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:5980:5: note: candidate: ‘template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Alloc> std::operator+(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, _CharT)’
     operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs, _CharT __rhs)
     ^~~~~~~~
/usr/include/c++/8/bits/basic_string.h:5980:5: note:   template argument deduction/substitution failed:
prog.cpp:6:66: note:   ‘main()::<lambda(auto:1)>’ is not derived from ‘const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>’
  auto bar = +[](auto a) { std::cout << "bar " << a << std::endl; };
                                                                  ^
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:5992:5: note: candidate: ‘template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Alloc> std::operator+(std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&&, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&)’
     operator+(basic_string<_CharT, _Traits, _Alloc>&& __lhs,
     ^~~~~~~~
/usr/include/c++/8/bits/basic_string.h:5992:5: note:   template argument deduction/substitution failed:
prog.cpp:6:66: note:   ‘main()::<lambda(auto:1)>’ is not derived from ‘std::__cxx11::basic_string<_CharT, _Traits, _Alloc>’
  auto bar = +[](auto a) { std::cout << "bar " << a << std::endl; };
                                                                  ^
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:5998:5: note: candidate: ‘template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Alloc> std::operator+(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&&)’
     operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
     ^~~~~~~~
/usr/include/c++/8/bits/basic_string.h:5998:5: note:   template argument deduction/substitution failed:
prog.cpp:6:66: note:   ‘main()::<lambda(auto:1)>’ is not derived from ‘const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>’
  auto bar = +[](auto a) { std::cout << "bar " << a << std::endl; };
                                                                  ^
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:6004:5: note: candidate: ‘template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Alloc> std::operator+(std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&&, std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&&)’
     operator+(basic_string<_CharT, _Traits, _Alloc>&& __lhs,
     ^~~~~~~~
/usr/include/c++/8/bits/basic_string.h:6004:5: note:   template argument deduction/substitution failed:
prog.cpp:6:66: note:   ‘main()::<lambda(auto:1)>’ is not derived from ‘std::__cxx11::basic_string<_CharT, _Traits, _Alloc>’
  auto bar = +[](auto a) { std::cout << "bar " << a << std::endl; };
                                                                  ^
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:6016:5: note: candidate: ‘template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Alloc> std::operator+(const _CharT*, std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&&)’
     operator+(const _CharT* __lhs,
     ^~~~~~~~
/usr/include/c++/8/bits/basic_string.h:6016:5: note:   template argument deduction/substitution failed:
prog.cpp:6:66: note:   mismatched types ‘const _CharT*’ and ‘main()::<lambda(auto:1)>’
  auto bar = +[](auto a) { std::cout << "bar " << a << std::endl; };
                                                                  ^
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:6022:5: note: candidate: ‘template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Alloc> std::operator+(_CharT, std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&&)’
     operator+(_CharT __lhs,
     ^~~~~~~~
/usr/include/c++/8/bits/basic_string.h:6022:5: note:   template argument deduction/substitution failed:
prog.cpp:6:66: note:   candidate expects 2 arguments, 1 provided
  auto bar = +[](auto a) { std::cout << "bar " << a << std::endl; };
                                                                  ^
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:6028:5: note: candidate: ‘template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Alloc> std::operator+(std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&&, const _CharT*)’
     operator+(basic_string<_CharT, _Traits, _Alloc>&& __lhs,
     ^~~~~~~~
/usr/include/c++/8/bits/basic_string.h:6028:5: note:   template argument deduction/substitution failed:
prog.cpp:6:66: note:   ‘main()::<lambda(auto:1)>’ is not derived from ‘std::__cxx11::basic_string<_CharT, _Traits, _Alloc>’
  auto bar = +[](auto a) { std::cout << "bar " << a << std::endl; };
                                                                  ^
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:6034:5: note: candidate: ‘template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Alloc> std::operator+(std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&&, _CharT)’
     operator+(basic_string<_CharT, _Traits, _Alloc>&& __lhs,
     ^~~~~~~~
/usr/include/c++/8/bits/basic_string.h:6034:5: note:   template argument deduction/substitution failed:
prog.cpp:6:66: note:   ‘main()::<lambda(auto:1)>’ is not derived from ‘std::__cxx11::basic_string<_CharT, _Traits, _Alloc>’
  auto bar = +[](auto a) { std::cout << "bar " << a << std::endl; };
                                                                  ^

Standard output is empty