How virtual keyword in virtual inheritance causing a error [duplicate]

I have two set of code :

First case :

#include<iostream>
using namespace std;
class A {
public:
    virtual void display() { cout << "A display \n"; }
};

class B : virtual public A {
public:
    void display() { cout << "B display \n"; }
};

class C : virtual public A {
public:
    void display() { cout << "C display \n"; }
};

class D : public C, public B {
public:
    D() {};
};

int main() {
    D d;
    d.B::display(); // ERROR
}

Error :

Error (active)  E0318   override of virtual function "A::display" is ambiguous
Error   C2250   'D': ambiguous inheritance of 'void A::display(void)'

Second case :

#include<iostream>
using namespace std;
class A {
public:
    void display() { cout << "A display \n"; }
};

class B : virtual public A {
public:
    void display() { cout << "B display \n"; }
};

class C : virtual public A {
public:
    void display() { cout << "C display \n"; }
};

class D : public C, public B {
public:
    D() {};
};

int main() {
    D d;
    d.B::display(); // Output B display
}

The only difference in above two cases is in first one I am making "virtual void display()" in class A, why it is failing in first case while passing in second case?