+// Special case std::pair to detect when both types are viable and when there
+// is no alignment-derived padding in the pair. This is a bit of a lie because
+// std::pair isn't truly POD, but it's close enough in all reasonable
+// implementations for our use case of hashing the underlying data.
+template <typename T, typename U> struct is_hashable_data<std::pair<T, U> >
+ : integral_constant<bool, (is_hashable_data<T>::value &&
+ is_hashable_data<U>::value &&
+ (sizeof(T) + sizeof(U)) ==
+ sizeof(std::pair<T, U>))> {};
+