foo.cc: In function 'int main()':
foo.cc:8:5: error: no match for 'operator<<' (operand types are 'std::istream' {aka 'std::basic_istream<char>'} and 'int')
    8 |  cin<<a;
      |  ~~~^~~
      |  |    |
      |  |    int
      |  std::istream {aka std::basic_istream<char>}
foo.cc:8:5: note: candidate: 'operator<<(int, int)' <built-in>
    8 |  cin<<a;
      |  ~~~^~~
foo.cc:8:5: note:   no known conversion for argument 1 from 'std::istream' {aka 'std::basic_istream<char>'} to 'int'
In file included from /usr/include/c++/9/string:55,
                 from /usr/include/c++/9/bits/locale_classes.h:40,
                 from /usr/include/c++/9/bits/ios_base.h:41,
                 from /usr/include/c++/9/ios:42,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/bits/basic_string.h:6416:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&)'
 6416 |     operator<<(basic_ostream<_CharT, _Traits>& __os,
      |     ^~~~~~~~
/usr/include/c++/9/bits/basic_string.h:6416:5: note:   template argument deduction/substitution failed:
foo.cc:8:7: note:   'std::istream' {aka 'std::basic_istream<char>'} is not derived from 'std::basic_ostream<_CharT, _Traits>'
    8 |  cin<<a;
      |       ^
In file included from /usr/include/c++/9/bits/ios_base.h:46,
                 from /usr/include/c++/9/ios:42,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/system_error:217:5: note: candidate: 'template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::error_code&)'
  217 |     operator<<(basic_ostream<_CharT, _Traits>& __os, const error_code& __e)
      |     ^~~~~~~~
/usr/include/c++/9/system_error:217:5: note:   template argument deduction/substitution failed:
foo.cc:8:7: note:   'std::istream' {aka 'std::basic_istream<char>'} is not derived from 'std::basic_ostream<_CharT, _Traits>'
    8 |  cin<<a;
      |       ^
In file included from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/ostream:506:5: note: candidate: 'template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, _CharT)'
  506 |     operator<<(basic_ostream<_CharT, _Traits>& __out, _CharT __c)
      |     ^~~~~~~~
/usr/include/c++/9/ostream:506:5: note:   template argument deduction/substitution failed:
foo.cc:8:7: note:   'std::istream' {aka 'std::basic_istream<char>'} is not derived from 'std::basic_ostream<_CharT, _Traits>'
    8 |  cin<<a;
      |       ^
In file included from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/ostream:511:5: note: candidate: 'template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, char)'
  511 |     operator<<(basic_ostream<_CharT, _Traits>& __out, char __c)
      |     ^~~~~~~~
/usr/include/c++/9/ostream:511:5: note:   template argument deduction/substitution failed:
foo.cc:8:7: note:   'std::istream' {aka 'std::basic_istream<char>'} is not derived from 'std::basic_ostream<_CharT, _Traits>'
    8 |  cin<<a;
      |       ^
In file included from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/ostream:517:5: note: candidate: 'template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, char)'
  517 |     operator<<(basic_ostream<char, _Traits>& __out, char __c)
      |     ^~~~~~~~
/usr/include/c++/9/ostream:517:5: note:   template argument deduction/substitution failed:
foo.cc:8:7: note:   'std::istream' {aka 'std::basic_istream<char>'} is not derived from 'std::basic_ostream<char, _Traits>'
    8 |  cin<<a;
      |       ^
In file included from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/ostream:523:5: note: candidate: 'template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, signed char)'
  523 |     operator<<(basic_ostream<char, _Traits>& __out, signed char __c)
      |     ^~~~~~~~
/usr/include/c++/9/ostream:523:5: note:   template argument deduction/substitution failed:
foo.cc:8:7: note:   'std::istream' {aka 'std::basic_istream<char>'} is not derived from 'std::basic_ostream<char, _Traits>'
    8 |  cin<<a;
      |       ^
In file included from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/ostream:528:5: note: candidate: 'template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, unsigned char)'
  528 |     operator<<(basic_ostream<char, _Traits>& __out, unsigned char __c)
      |     ^~~~~~~~
/usr/include/c++/9/ostream:528:5: note:   template argument deduction/substitution failed:
foo.cc:8:7: note:   'std::istream' {aka 'std::basic_istream<char>'} is not derived from 'std::basic_ostream<char, _Traits>'
    8 |  cin<<a;
      |       ^
In file included from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/ostream:548:5: note: candidate: 'template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const _CharT*)'
  548 |     operator<<(basic_ostream<_CharT, _Traits>& __out, const _CharT* __s)
      |     ^~~~~~~~
/usr/include/c++/9/ostream:548:5: note:   template argument deduction/substitution failed:
foo.cc:8:7: note:   'std::istream' {aka 'std::basic_istream<char>'} is not derived from 'std::basic_ostream<_CharT, _Traits>'
    8 |  cin<<a;
      |       ^
In file included from /usr/include/c++/9/ostream:702,
                 from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/bits/ostream.tcc:321:5: note: candidate: 'template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const char*)'
  321 |     operator<<(basic_ostream<_CharT, _Traits>& __out, const char* __s)
      |     ^~~~~~~~
/usr/include/c++/9/bits/ostream.tcc:321:5: note:   template argument deduction/substitution failed:
foo.cc:8:7: note:   'std::istream' {aka 'std::basic_istream<char>'} is not derived from 'std::basic_ostream<_CharT, _Traits>'
    8 |  cin<<a;
      |       ^
In file included from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/ostream:565:5: note: candidate: 'template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const char*)'
  565 |     operator<<(basic_ostream<char, _Traits>& __out, const char* __s)
      |     ^~~~~~~~
/usr/include/c++/9/ostream:565:5: note:   template argument deduction/substitution failed:
foo.cc:8:7: note:   'std::istream' {aka 'std::basic_istream<char>'} is not derived from 'std::basic_ostream<char, _Traits>'
    8 |  cin<<a;
      |       ^
In file included from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/ostream:578:5: note: candidate: 'template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const signed char*)'
  578 |     operator<<(basic_ostream<char, _Traits>& __out, const signed char* __s)
      |     ^~~~~~~~
/usr/include/c++/9/ostream:578:5: note:   template argument deduction/substitution failed:
foo.cc:8:7: note:   'std::istream' {aka 'std::basic_istream<char>'} is not derived from 'std::basic_ostream<char, _Traits>'
    8 |  cin<<a;
      |       ^
In file included from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/ostream:583:5: note: candidate: 'template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const unsigned char*)'
  583 |     operator<<(basic_ostream<char, _Traits>& __out, const unsigned char* __s)
      |     ^~~~~~~~
/usr/include/c++/9/ostream:583:5: note:   template argument deduction/substitution failed:
foo.cc:8:7: note:   'std::istream' {aka 'std::basic_istream<char>'} is not derived from 'std::basic_ostream<char, _Traits>'
    8 |  cin<<a;
      |       ^
In file included from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/ostream:691:5: note: candidate: 'template<class _Ostream, class _Tp> typename std::enable_if<std::__and_<std::__not_<std::is_lvalue_reference<_Tp> >, std::__is_convertible_to_basic_ostream<_Ostream>, std::__is_insertable<typename std::__is_convertible_to_basic_ostream<_Tp>::__ostream_type, const _Tp&, void> >::value, typename std::__is_convertible_to_basic_ostream<_Tp>::__ostream_type>::type std::operator<<(_Ostream&&, const _Tp&)'
  691 |     operator<<(_Ostream&& __os, const _Tp& __x)
      |     ^~~~~~~~
/usr/include/c++/9/ostream:691:5: note:   template argument deduction/substitution failed:
/usr/include/c++/9/ostream: In substitution of 'template<class _Ostream, class _Tp> typename std::enable_if<std::__and_<std::__not_<std::is_lvalue_reference<_Tp> >, std::__is_convertible_to_basic_ostream<_Ostream>, std::__is_insertable<typename std::__is_convertible_to_basic_ostream<_Tp>::__ostream_type, const _Tp&, void> >::value, typename std::__is_convertible_to_basic_ostream<_Tp>::__ostream_type>::type std::operator<<(_Ostream&&, const _Tp&) [with _Ostream = std::basic_istream<char>&; _Tp = int]':
foo.cc:8:7:   required from here
/usr/include/c++/9/ostream:691:5: error: no type named 'type' in 'struct std::enable_if<false, void>'
foo.cc:9:3: warning: statement has no effect [-Wunused-value]
    9 |  a*a;
      |  ~^~
foo.cc: At global scope:
foo.cc:12:1: error: expected unqualified-id before 'return'
   12 | return 0;
      | ^~~~~~

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