Why is cast from NULL illegal?

[Lars Clausen <lrclause cs uiuc edu>]

After changing Dia's code to have more static type checking, I'm getting a
number of errors about invalid casts from NULL.  But surely NULL can be any
type, so any cast of it should be valid.

The function in question here is gtk_item_factory_get_item(), which despite
always returning NULL or a GtkMenuItem* has a return type of GtkWidget*.
After some nasty bugs caused by the lack of static type check for this, I
decided to make better use of the compilers typechecking.  But if I say
GTK_MENU_ITEM(gtk_item_factory_get_item(...)), I get warnings whenever it
returns NULL.  Having to check for NULL before casting would be
cumbersome.  

So: Is there any good reason why the (or some) type check macros don't
consider NULL to be of their type?

-Lars

-- 
Lars Clausen (http://shasta.cs.uiuc.edu/~lrclause)| Hårdgrim of Numenor
"I do not agree with a word that you say, but I   |----------------------------
will defend to the death your right to say it."   | Where are we going, and
    --Evelyn Beatrice Hall paraphrasing Voltaire  | what's with the handbasket?