What's the difference between "= default" destructor and empty destructor?

I want to prevent the user of my class from using it as an automatic variable, so I write code like this:

class A {
private:
  ~A() = default;
};

int main() {
  A a;
}

I expect that the code won't be compiled, but g++ compiles it without error.

However, when I change the code to:

class A {
private:
  ~A(){}
};

int main() {
  A a;
}

Now, g++ gives the error that ~A() is private, as is my expectation.

What's the difference between a "= default" destructor and an empty destructor?