Multiple inheritance ambiguous base class

Question

Consider the code

struct Base{};
struct Derived: public Base{};

struct A: public Base{};

struct B: public A, public Base{};

struct C: public A, public Derived{}; // why no ambiguity here?

int main() {}

The compiler (g++5.1) warns that

warning: direct base 'Base' inaccessible in 'B' due to ambiguity struct B: public A, public Base{};

I understand this, Base is duplicated in B.

1. Why is there no warning for C? Doesn't C inherit from both A and Derived, which both inherit from Base?

2. Why adding virtual

   struct Derived: virtual Base{};
   

results now in both B and C emitting warnings, live on Wandbox

warning: direct base 'Base' inaccessible in 'B' due to ambiguity struct B: public A, public Base{};

warning: direct base 'Base' inaccessible in 'C' due to ambiguity struct C: public A, public Derived{};

Answer 1

In B, it's impossible to refer to members of the Base subobject inherited directly. Consider:

struct Base {
    int x;
};

struct B: public A, public Base {
    void foo() {
        int& x1 = A::x; // OK
        int& x2 = x; // ambiguous
        // no way to refer to the x in the direct base
    }
};

In C this is not a problem. Both x's can be referred to using qualified names:

struct C: public A, public Derived {
    void foo() {
        int& x1 = A::x; // OK
        int& x2 = Derived::x; // OK
    }
};

So the warning you get is one that only makes sense when a direct base is also inherited through another path.

For your second question, I couldn't reproduce the warning with C on Coliru with g++-5.1.