using typedefed types as covariant return types

[Rick Mann]

I was hoping I could do this:

typedef void* CFTypeRef;
typedef CFTypeRef CFStringRef;

class DCFObject
{
  CFTypeRef getRef() {}
}

class DString : DCFObject
{
  CFStringRef getRef() {}
}

But the compiler gives me:

src/d/darbon/corefoundation/DString.d:32: function darbon.corefoundation.DString.DString.getRef of type CFStringRef() overrides but is not covariant with darbon.corefoundation.DCFObject.DCFObject.getRef of type CFTypeRef()