Trying to do something simple like this:
---

public static int main (string[] args) {
    Object obj = new Object ();
    ObjectClass cclass = obj.get_class ();

    return 0;
}

---
Generates buggy C code with compiler warnings like this:
---

/tmp/type.vala.c: In function '_g_type_class_ref0':
/tmp/type.vala.c:19:2: warning: passing argument 1 of 'g_type_class_ref' makes integer from pointer without a cast [enabled by default]
In file included from /usr/include/glib-2.0/gobject/gobject.h:26:0,
                 from /usr/include/glib-2.0/gobject/gbinding.h:31,
                 from /usr/include/glib-2.0/glib-object.h:25,
                 from /tmp/type.vala.c:6:
/usr/include/glib-2.0/gobject/gtype.h:678:23: note: expected 'GType' but argument is of type 'gpointer'
Compilation succeeded - 1 warning(s)

---
With C code like this:
---

        obj = _tmp2_;
        _tmp3_ = G_OBJECT_GET_CLASS (obj);
        _tmp4_ = _g_type_class_ref0 (_tmp3_);
        cclass = _tmp4_;

---
Where _g_type_class_ref0 is vala-generated:
---

static gpointer _g_type_class_ref0 (gpointer self) {
        return self ? g_type_class_ref (self) : NULL;
}

---
which results in a runtime error like this
---

(process:3119): GLib-GObject-WARNING **: cannot retrieve class for invalid (unclassed) type `(null)'

---
So the problem seems to be that we get a GObjectClass in _tmp3_ and then we treat it like a GType in _g_type_class_ref0.

The wonderful nemequ in #vala has suggested as a work-around saving the GLib.ObjectClass into an unowned variable for now.