Currently we don't allow (allow-none) for the return value, e.g. I think we just implicitly assume that the return value can always be NULL.

While this is probably the right default, it would be useful to be able to specify that a function can never return NULL. This applies to e.g. g_object_new(). Another example

 /**
  * foo_bar_get_baz:
  * @bar: A #FooBar.
  *
  * Gets the #FooBaz object associated with @bar.
  *
  * Returns: (never-none): A #FooBaz. Free with g_object_unref().
  */
 FooBaz *
 foo_bar_get_baz (FooBar *bar)
 {
   return bar->priv->baz;
 }

This is useful in at least a couple of cases

 - Bindings can handle the cases earlier where a method returns NULL and it 
   shouldn't

 - Things like C++ can return a reference instead of a pointer
   (however I'm not sure this even a good idea - then again, my C++
    knowledge is rather rusty)

However, maybe this is not useful at all. But I thought I'd file a bug anyway.