Why the size of empty class that is derived from two empty classes is 2?

Question

What I know is that size of empty class is 1 just to conform to the standard that does not allow objects (and classes thereof) of size to be 0. Here, I am getting size of a derived class D as 2. Why behavior is different in this case given that there is no data member or virtual pointer inherited from class B and C to D?

#include<iostream>
using namespace std;

class A {};
class B : public A {};
class C : public A {};
class D : public B, public C {};

int main() {
    cout<<"Size of D is: "<<sizeof(D)<<endl;
    return 0;
}

Answer 1

To me it seems that whether or not the empty base optimization can be applied here, depends on how one interprets [intro.object/8]:

Unless an object is a bit-field or a base class subobject of zero size, the address of that object is the address of the first byte it occupies. Two objects a and b with overlapping lifetimes that are not bit-fields may have the same address if one is nested within the other, or if at least one is a base class subobject of zero size and they are of different types; otherwise, they have distinct addresses.

Are B and C different types? They both are an A as well. Two distinct A objects actually. A compiler writer is allowed to stop right there an allocate storage for B and C separately, without checking that A is empty.

It's worth noting that with g++ the size is back to 1 if you have B and C inherit from separate bases:

Live Example

#include<iostream>

class A  {};
class A1 {};
class B : public A {};
class C : public A1 {};
class D : public B, public C {};

int main() {
    std::cout << "Size of D is: " << sizeof(D) << std::endl;
    return 0;
}