Getting "source type is not polymorphic" when trying to use dynamic_cast

You need at least a virtual function - typically, if no others are suitable, the destructor:

struct A {
   virtual ~A() {}
}; 

You need to make A polymorphic, which you can do by adding a virtual destructor or any virtual function:

struct A {
  virtual ~A() = default;
};

or, before C++11,

struct A {
  virtual ~A() {}
};

Note that a polymorphic type should have a virtual destructor anyway, if you intend to safely call delete on instances of a derived type via a pointer to the base.

As your compiler says, your type A is not polymorphic. You should add a virtual function to it. For instance, a virtual destructor could be a good choice:

struct A { virtual ~A() { } };
//         ^^^^^^^ This makes A a polymorphic type

struct B : A {};

int main()
{
    A* a = new B();

    B* b = dynamic_cast<B*>(a); // Should work now
}