/in/foo.cc:76:13: error: specialization of 'template<class _Tp> struct std::hash' in different namespace [-fpermissive]
 struct std::hash<std::pair<std::pair<int, int>, int>> {
             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/6/bits/basic_string.h:5643:0,
                 from /usr/include/c++/6/string:52,
                 from /usr/include/c++/6/bits/locale_classes.h:40,
                 from /usr/include/c++/6/bits/ios_base.h:41,
                 from /usr/include/c++/6/ios:42,
                 from /usr/include/c++/6/istream:38,
                 from /usr/include/c++/6/sstream:38,
                 from /usr/include/c++/6/complex:45,
                 from /usr/include/c++/6/ccomplex:39,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:52,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/functional_hash.h:58:12: error:   from definition of 'template<class _Tp> struct std::hash' [-fpermissive]
     struct hash;
            ^~~~