No implicitly convert derived ptr to base ptr?

[Timon Gehr]

On 04/25/2012 07:50 AM, Nick Sabalausky wrote:
> The compiler rejects this:
>
>      class Base {}
>      class Derived : Base {}
>
>      void main()
>      {
>          Base*    basePtr;
>          Derived* derivedPtr;
>
>          basePtr = derivedPtr; // ERROR
>      }
>
> Is that really correct that it shouldn't be allowed?
>
>

Yes it is. It was allowed a number of releases ago, but that bug has 
since been fixed.

Example that shows unsoundness:

class Base {}
class Derived1 : Base {}
class Derived2 : Base {}

void main() {
     Derived1 d1;
     Base* basePtr = &d1;
     *basePtr = new Derived2;
     assert(typeid(d1)==typeid(Derived2) && !is(Derived2: typeof(d1));
}


Note that the conversion succeeds if the tail of the pointer is not 
mutable, eg Derived* implicitly converts to const(Base)*.