In file included from /usr/include/c++/6/bits/hashtable.h:35:0,
                 from /usr/include/c++/6/unordered_map:47,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:117,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/hashtable_policy.h: In instantiation of 'struct std::__detail::__is_noexcept_hash<std::pair<int, int>, std::hash<std::pair<int, int> > >':
/usr/include/c++/6/type_traits:143:12:   required from 'struct std::__and_<std::__is_fast_hash<std::hash<std::pair<int, int> > >, std::__detail::__is_noexcept_hash<std::pair<int, int>, std::hash<std::pair<int, int> > > >'
/usr/include/c++/6/type_traits:154:38:   required from 'struct std::__not_<std::__and_<std::__is_fast_hash<std::hash<std::pair<int, int> > >, std::__detail::__is_noexcept_hash<std::pair<int, int>, std::hash<std::pair<int, int> > > > >'
/usr/include/c++/6/bits/unordered_set.h:95:63:   required from 'class std::unordered_set<std::pair<int, int> >'
/in/foo.cc:46:32:   required from here
/usr/include/c++/6/bits/hashtable_policy.h:85:34: error: no match for call to '(const std::hash<std::pair<int, int> >) (const std::pair<int, int>&)'
  noexcept(declval<const _Hash&>()(declval<const _Key&>()))>
           ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/6/bits/move.h:57:0,
                 from /usr/include/c++/6/bits/nested_exception.h:40,
                 from /usr/include/c++/6/exception:173,
                 from /usr/include/c++/6/ios:39,
                 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/type_traits: In instantiation of 'struct std::__not_<std::__and_<std::__is_fast_hash<std::hash<std::pair<int, int> > >, std::__detail::__is_noexcept_hash<std::pair<int, int>, std::hash<std::pair<int, int> > > > >':
/usr/include/c++/6/bits/unordered_set.h:95:63:   required from 'class std::unordered_set<std::pair<int, int> >'
/in/foo.cc:46:32:   required from here
/usr/include/c++/6/type_traits:154:38: error: 'value' is not a member of 'std::__and_<std::__is_fast_hash<std::hash<std::pair<int, int> > >, std::__detail::__is_noexcept_hash<std::pair<int, int>, std::hash<std::pair<int, int> > > >'
     : public integral_constant<bool, !_Pp::value>
                                      ^~~~
In file included from /usr/include/c++/6/unordered_set:48:0,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:118,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/unordered_set.h: In instantiation of 'class std::unordered_set<std::pair<int, int> >':
/in/foo.cc:46:32:   required from here
/usr/include/c++/6/bits/unordered_set.h:95:63: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<std::hash<std::pair<int, int> > >, std::__detail::__is_noexcept_hash<std::pair<int, int>, std::hash<std::pair<int, int> > > > >'
       typedef __uset_hashtable<_Value, _Hash, _Pred, _Alloc>  _Hashtable;
                                                               ^~~~~~~~~~
/usr/include/c++/6/bits/unordered_set.h:102:45: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<std::hash<std::pair<int, int> > >, std::__detail::__is_noexcept_hash<std::pair<int, int>, std::hash<std::pair<int, int> > > > >'
       typedef typename _Hashtable::key_type key_type;
                                             ^~~~~~~~
/usr/include/c++/6/bits/unordered_set.h:103:47: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<std::hash<std::pair<int, int> > >, std::__detail::__is_noexcept_hash<std::pair<int, int>, std::hash<std::pair<int, int> > > > >'
       typedef typename _Hashtable::value_type value_type;
                                               ^~~~~~~~~~
/usr/include/c++/6/bits/unordered_set.h:104:43: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<std::hash<std::pair<int, int> > >, std::__detail::__is_noexcept_hash<std::pair<int, int>, std::hash<std::pair<int, int> > > > >'
       typedef typename _Hashtable::hasher hasher;
                                           ^~~~~~
/usr/include/c++/6/bits/unordered_set.h:105:46: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<std::hash<std::pair<int, int> > >, std::__detail::__is_noexcept_hash<std::pair<int, int>, std::hash<std::pair<int, int> > > > >'
       typedef typename _Hashtable::key_equal key_equal;
                                              ^~~~~~~~~
/usr/include/c++/6/bits/unordered_set.h:106:51: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<std::hash<std::pair<int, int> > >, std::__detail::__is_noexcept_hash<std::pair<int, int>, std::hash<std::pair<int, int> > > > >'
       typedef typename _Hashtable::allocator_type allocator_type;
                                                   ^~~~~~~~~~~~~~
/usr/include/c++/6/bits/unordered_set.h:111:45: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<std::hash<std::pair<int, int> > >, std::__detail::__is_noexcept_hash<std::pair<int, int>, std::hash<std::pair<int, int> > > > >'
       typedef typename _Hashtable::pointer  pointer;
                                             ^~~~~~~
/usr/include/c++/6/bits/unordered_set.h:112:50: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<std::hash<std::pair<int, int> > >, std::__detail::__is_noexcept_hash<std::pair<int, int>, std::hash<std::pair<int, int> > > > >'
       typedef typename _Hashtable::const_pointer const_pointer;
                                                  ^~~~~~~~~~~~~
/usr/include/c++/6/bits/unordered_set.h:113:47: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<std::hash<std::pair<int, int> > >, std::__detail::__is_noexcept_hash<std::pair<int, int>, std::hash<std::pair<int, int> > > > >'
       typedef typename _Hashtable::reference  reference;
                                               ^~~~~~~~~
/usr/include/c++/6/bits/unordered_set.h:114:52: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<std::hash<std::pair<int, int> > >, std::__detail::__is_noexcept_hash<std::pair<int, int>, std::hash<std::pair<int, int> > > > >'
       typedef typename _Hashtable::const_reference const_reference;
                                                    ^~~~~~~~~~~~~~~
/usr/include/c++/6/bits/unordered_set.h:115:46: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<std::hash<std::pair<int, int> > >, std::__detail::__is_noexcept_hash<std::pair<int, int>, std::hash<std::pair<int, int> > > > >'
       typedef typename _Hashtable::iterator  iterator;
                                              ^~~~~~~~
/usr/include/c++/6/bits/unordered_set.h:116:51: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<std::hash<std::pair<int, int> > >, std::__detail::__is_noexcept_hash<std::pair<int, int>, std::hash<std::pair<int, int> > > > >'
       typedef typename _Hashtable::const_iterator const_iterator;
                                                   ^~~~~~~~~~~~~~
/usr/include/c++/6/bits/unordered_set.h:117:51: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<std::hash<std::pair<int, int> > >, std::__detail::__is_noexcept_hash<std::pair<int, int>, std::hash<std::pair<int, int> > > > >'
       typedef typename _Hashtable::local_iterator local_iterator;
                                                   ^~~~~~~~~~~~~~
/usr/include/c++/6/bits/unordered_set.h:118:57: error: 'value' is not a member of 'std::__not_<std::__and_<std::__is_fast_hash<std::hash<std::pair<int, int> > >, std::__detail::__is_noexcept_hash<std::pair<int, int>, std::hash<std::pair<int, int> > > > >'
       typedef typename _Hashtable::const_local_iterator const_local_iterator;
                                                         ^~~~~~~~~~~~~~~