Trouble with unordered_map pair index

Question

I have the following code:

#include <unordered_map>
#include <utility>

typedef std::pair<int, int> rc;
std::unordered_map<rc, int> all_cells;

but the compiler is complaining:

/usr/include/c++/7.1.1/bits/hashtable_policy.h:87:34: 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&>()))>

What gives?