Why does hash template specialization require result_type and argument_type?

Why does C++11 require a hash template specialization to define result_type and argument_type? I found some old code where a user-defined type is successfully used for an unordered_map<> key but doesn't have these two typedef's.

namespace std {
  template<> struct hash<MyNamespace::Foo> {
    typedef size_t result_type;
    typedef MyNamespace::Foo argument_type;
    size_t operator() (const MyNamespace::Foo& f ) const { return (size_t) f.u32InternalValue; }
  };
}