/in/foo.cc:2:1: error: expected unqualified-id before numeric constant
 3
 ^
In file included from /usr/include/c++/6/cmath:43:0,
                 from /in/foo.cc:16:
/usr/include/c++/6/ext/type_traits.h:162:35: error: '__gnu_cxx::__is_null_pointer' declared as an 'inline' variable
   __is_null_pointer(std::nullptr_t)
                                   ^
/usr/include/c++/6/ext/type_traits.h:162:35: error: 'bool __gnu_cxx::__is_null_pointer' redeclared as different kind of symbol
/usr/include/c++/6/ext/type_traits.h:157:5: note: previous declaration 'template<class _Type> bool __gnu_cxx::__is_null_pointer(_Type)'
     __is_null_pointer(_Type)
     ^~~~~~~~~~~~~~~~~
/usr/include/c++/6/ext/type_traits.h:162:21: error: 'nullptr_t' is not a member of 'std'
   __is_null_pointer(std::nullptr_t)
                     ^~~