/in/foo.cc: In function 'int main()':
/in/foo.cc:9:11: error: no match for 'operator<' (operand types are 'std::basic_ostream<char>::__ostream_type {aka std::basic_ostream<char>}' and '<unresolved overloaded function type>')
  cout<<a+b<endl;
           ^
/in/foo.cc:9:11: note: candidates are:
In file included from /usr/include/c++/4.8.2/bits/stl_algobase.h:64:0,
                 from /usr/include/c++/4.8.2/bits/char_traits.h:39,
                 from /usr/include/c++/4.8.2/ios:40,
                 from /usr/include/c++/4.8.2/ostream:38,
                 from /usr/include/c++/4.8.2/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/4.8.2/bits/stl_pair.h:220:5: note: 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++/4.8.2/bits/stl_pair.h:220:5: note:   template argument deduction/substitution failed: