Well, not everybody could do it just from the forward declared class. Yes, nsCOMPtr.h and nsIWeakReference.h include eachother, but it works anyway (they test for already being included).

xpcom/base/nsCOMPtr.h

@@ -32,6 +32,11 @@
   // for |nsresult|, |NS_ADDREF|, et al
 #endif
 
+#ifndef __gen_nsIWeakReference_h__
+#include "nsIWeakReference.h"
+	// for |nsIWeakReference|
+#endif
+
 /*
   Public things defined in this file:
 
@@ -169,8 +174,6 @@
   #define NSCAP_RELEASE(ptr)   (ptr)->Release()
 #endif
 
-class nsIWeakReference;
-
   /*
     WARNING:
       VC++4.2 is very picky.  To compile under VC++4.2, the classes must be defined

xpcom/glue/nsCOMPtr.h

@@ -32,6 +32,11 @@
   // for |nsresult|, |NS_ADDREF|, et al
 #endif
 
+#ifndef __gen_nsIWeakReference_h__
+#include "nsIWeakReference.h"
+	// for |nsIWeakReference|
+#endif
+
 /*
   Public things defined in this file:
 
@@ -169,8 +174,6 @@
   #define NSCAP_RELEASE(ptr)   (ptr)->Release()
 #endif
 
-class nsIWeakReference;
-
   /*
     WARNING:
       VC++4.2 is very picky.  To compile under VC++4.2, the classes must be defined