问为什么C++11允许抽象的最终类？

EN

struct Base {
  virtual void foo() = 0;
};

// since this class is final and abstract, it can never be
// instantiated - why isn't its very declaration an error?
struct Derived final : Base {};

int main() {
  //Derived derived; // this IS an error, but relies on someone trying
                     // to instantiate the class, and the error is at the site
                     // of instantiation, not the class itself
}