Using typedefed types as covariant return types

[Rick Mann]

I didn't get any responses in D.learn for this question, so I decided maybe it was advanced enough for this group.

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()

>From what I gather in the docs, CFStringRef is derived from CFTypeRef, so this should be acceptable. Am I missing something?

TIA,
Rick