namespace a {
  namespace b {
    template<class T>
    struct qualifies : std::false_type {};
  }
}

namespace other {
  struct bob {};
}

namespace a::b {
  template<>
  struct qualifies<::other::bob> : std::true_type {};
}

You can enter both the a and b namespaces in one step with namespace a::b starting in C++17.