/in/foo.cc: In function 'int main()':
/in/foo.cc:87:20: error: reference to 'hash' is ambiguous
         int code = hash(o);
                    ^
/in/foo.cc:29:5: note: candidates are: int hash(int)
 int hash(int x)
     ^
In file included from /usr/include/c++/5/bits/basic_string.h:5471:0,
                 from /usr/include/c++/5/string:52,
                 from /usr/include/c++/5/bits/locale_classes.h:40,
                 from /usr/include/c++/5/bits/ios_base.h:41,
                 from /usr/include/c++/5/ios:42,
                 from /usr/include/c++/5/ostream:38,
                 from /usr/include/c++/5/iostream:39,
                 from /in/foo.cc:3:
/usr/include/c++/5/bits/functional_hash.h:58:12: note:                 template<class _Tp> struct std::hash
     struct hash;
            ^