I always thought that when I converted a member function ptr in C to a member function ptr in A, I just had to make sure that the call was pointing to an C entity , but that was a mistake. Does the standard describe this use case in more detail?(checked with multiple compilers)

struct A
{
    char i[7];
    void test1(){
        std::cout << __LINE__;
    }
};

struct B
{
    char i[14];
    virtual void test(){
        std::cout << __LINE__;
    }
};

struct C:A,B
{
    char i[31];
};

using T = void (C::*)();

int main()
{
    C c;
    (c.*T(&B::test))();//good
    (c.*static_cast<decltype(&A::test1)>(T(&B::test)))();//crash
    return 0;
} 

If that’s the case, why do member function pointers for polymorphic classes require two pointers’ worth of space? Can’t the offset of vtable_ptr be computed through the type?”