/in/foo.cc: In function 'int main()':
/in/foo.cc:7:5: error: no match for 'operator>>' (operand types are 'std::ostream {aka std::basic_ostream<char>}' and 'int')
 cout>>a+b;
 ~~~~^~~~~
/in/foo.cc:7:5: note: candidate: operator>>(int, int) <built-in>
/in/foo.cc:7:5: note:   no known conversion for argument 1 from 'std::ostream {aka std::basic_ostream<char>}' to 'int'
In file included from /usr/include/c++/7/string:53:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/7/bits/basic_string.tcc:1465:5: note: candidate: template<class _CharT, class _Traits, class _Alloc> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&)
     operator>>(basic_istream<_CharT, _Traits>& __in,
     ^~~~~~~~
/usr/include/c++/7/bits/basic_string.tcc:1465:5: note:   template argument deduction/substitution failed:
/in/foo.cc:7:9: note:   'std::ostream {aka std::basic_ostream<char>}' is not derived from 'std::basic_istream<_CharT, _Traits>'
 cout>>a+b;
         ^
In file included from /usr/include/c++/7/istream:991:0,
                 from /usr/include/c++/7/iostream:40,
                 from /in/foo.cc:1:
/usr/include/c++/7/bits/istream.tcc:963:5: note: candidate: template<class _CharT2, class _Traits2> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, _CharT2*)
     operator>>(basic_istream<_CharT, _Traits>& __in, _CharT* __s)
     ^~~~~~~~
/usr/include/c++/7/bits/istream.tcc:963:5: note:   template argument deduction/substitution failed:
/in/foo.cc:7:9: note:   'std::ostream {aka std::basic_ostream<char>}' is not derived from 'std::basic_istream<_CharT, _Traits>'
 cout>>a+b;
         ^
In file included from /usr/include/c++/7/istream:991:0,
                 from /usr/include/c++/7/iostream:40,
                 from /in/foo.cc:1:
/usr/include/c++/7/bits/istream.tcc:931:5: note: candidate: template<class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&, _CharT&)
     operator>>(basic_istream<_CharT, _Traits>& __in, _CharT& __c)
     ^~~~~~~~
/usr/include/c++/7/bits/istream.tcc:931:5: note:   template argument deduction/substitution failed:
/in/foo.cc:7:9: note:   'std::ostream {aka std::basic_ostream<char>}' is not derived from 'std::basic_istream<_CharT, _Traits>'
 cout>>a+b;
         ^
In file included from /usr/include/c++/7/iostream:40:0,
                 from /in/foo.cc:1:
/usr/include/c++/7/istream:756:5: note: candidate: template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(std::basic_istream<char, _Traits>&, unsigned char&)
     operator>>(basic_istream<char, _Traits>& __in, unsigned char& __c)
     ^~~~~~~~
/usr/include/c++/7/istream:756:5: note:   template argument deduction/substitution failed:
/in/foo.cc:7:9: note:   'std::ostream {aka std::basic_ostream<char>}' is not derived from 'std::basic_istream<char, _Traits>'
 cout>>a+b;
         ^
In file included from /usr/include/c++/7/iostream:40:0,
                 from /in/foo.cc:1:
/usr/include/c++/7/istream:761:5: note: candidate: template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(std::basic_istream<char, _Traits>&, signed char&)
     operator>>(basic_istream<char, _Traits>& __in, signed char& __c)
     ^~~~~~~~
/usr/include/c++/7/istream:761:5: note:   template argument deduction/substitution failed:
/in/foo.cc:7:9: note:   'std::ostream {aka std::basic_ostream<char>}' is not derived from 'std::basic_istream<char, _Traits>'
 cout>>a+b;
         ^
In file included from /usr/include/c++/7/iostream:40:0,
                 from /in/foo.cc:1:
/usr/include/c++/7/istream:803:5: note: candidate: template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(std::basic_istream<char, _Traits>&, unsigned char*)
     operator>>(basic_istream<char, _Traits>& __in, unsigned char* __s)
     ^~~~~~~~
/usr/include/c++/7/istream:803:5: note:   template argument deduction/substitution failed:
/in/foo.cc:7:9: note:   'std::ostream {aka std::basic_ostream<char>}' is not derived from 'std::basic_istream<char, _Traits>'
 cout>>a+b;
         ^
In file included from /usr/include/c++/7/iostream:40:0,
                 from /in/foo.cc:1:
/usr/include/c++/7/istream:808:5: note: candidate: template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(std::basic_istream<char, _Traits>&, signed char*)
     operator>>(basic_istream<char, _Traits>& __in, signed char* __s)
     ^~~~~~~~
/usr/include/c++/7/istream:808:5: note:   template argument deduction/substitution failed:
/in/foo.cc:7:9: note:   'std::ostream {aka std::basic_ostream<char>}' is not derived from 'std::basic_istream<char, _Traits>'
 cout>>a+b;
         ^
In file included from /usr/include/c++/7/iostream:40:0,
                 from /in/foo.cc:1:
/usr/include/c++/7/istream:980:5: note: candidate: template<class _Istream, class _Tp> typename std::enable_if<std::__and_<std::__not_<std::is_lvalue_reference<_Tp> >, std::__is_convertible_to_basic_istream<_Istream>, std::__is_extractable<typename std::__is_convertible_to_basic_istream<_Tp>::__istream_type, _Tp&&, void> >::value, typename std::__is_convertible_to_basic_istream<_Tp>::__istream_type>::type std::operator>>(_Istream&&, _Tp&&)
     operator>>(_Istream&& __is, _Tp&& __x)
     ^~~~~~~~
/usr/include/c++/7/istream:980:5: note:   template argument deduction/substitution failed:
/usr/include/c++/7/istream: In substitution of 'template<class _Istream, class _Tp> typename std::enable_if<std::__and_<std::__not_<std::is_lvalue_reference<_Tp> >, std::__is_convertible_to_basic_istream<_Istream>, std::__is_extractable<typename std::__is_convertible_to_basic_istream<_Tp>::__istream_type, _Tp&&, void> >::value, typename std::__is_convertible_to_basic_istream<_Tp>::__istream_type>::type std::operator>>(_Istream&&, _Tp&&) [with _Istream = std::basic_ostream<char>&; _Tp = int]':
/in/foo.cc:7:9:   required from here
/usr/include/c++/7/istream:980:5: error: no type named 'type' in 'struct std::enable_if<false, void>'
/in/foo.cc:7:8: warning: suggest parentheses around '+' inside '>>' [-Wparentheses]
 cout>>a+b;
       ~^~