Design consideration of C++ dynamic_cast of also examining the pointer or reference type

Question

When we are doing dynamic cast:

A* x = ...;
B* b = dynamic_cast<B*>(x);

The dynamic_cast will return valid pointer only when:

1. A is polymorphic, otherwise compilation fails.
2. B is equivalent to A or is derived from A B should have a relationship with A in inheritance hierarchy (although I am not sure about the strategy), otherwise returns nullptr. This is updated according to the comments.
3. The RTTI of *x shows that it is an object of class B or a derived class of B, otherwise returns nullptr.

I am considering the case where both condition 1 and condition 3 are met, except condition 2. This can be possible because of memory copy or reinterpret_cast, etc. All following discussions are based on this scenario.

One little example is here: http://ideone.com/tBctgT

Can I say that: If in such situation did C++ allow the dynamic_cast to be successful, it would still be safe to use the pointer returned by dynamic_cast? If so, why C++ standard determines that the validation of condition 2 is mandatory?

In addition, condition 2 is possible to be examined at compile time. Why C++ standard determines to return nullptr at run-time instead of giving compilation error if it is not considered a proper operation?

Answer 1

I think an example is the simplest here:

class A { ... };
class B : public A { ... };
class C : public A { ... };

A *x = new C;
B *y = dynamic_cast<B *>(x);

Point 1.

A is a class, it is polymorphic.

Point 2.

The dynamic_cast<>() is allowed at compile time because B derives from A and x is of type A *.

Point 3.

The dynamic_cast<>() returns null because x does not represent an object of type B (or a class that derives from B.) So the code fails at runtime and you get y == nullptr.