TypeInfo of interfaces bugg or feature?

[TheFlyingFiddle]

When using the typeid expression on a instance of an interface it 
gives the type of the interface not the type of the underlying 
object. Is this intended or is it a bug?

interface I { }

class A : I { }
class B : A { }

unittest
{
   A a = new A();
   B b = new B();

   writeln(typeid(a)); //Prints A
   writeln(typeid(b)); //Prints B
   a = b;
   writeln(typeid(b)); //Prints B

   I i = b;
   writeln(typeid(i)); //Prints I ????????
}