The following code:

struct Test {
    int a;
}
class Master : GLib.Object {
    public virtual Test prop { get; set; }
}

class Child : Master {
    public override Test prop {
        get {
            return base.prop;
        }
        set {
            base.prop = value;
        }
    }
}

static void
main ()
{
    Child child = new Child ();
    child.prop.a = 10;
}

generates invalid C code which fails on build with following error:

test.vala.c: In function ‘child_real_get_prop’:
test.vala.c:15:29: error: too few arguments to function ‘((MasterClass *)g_type_check_class_cast((GTypeClass *)child_parent_class,  master_get_type()))->get_prop’
 #define MASTER_CLASS(klass) (G_TYPE_CHECK_CLASS_CAST ((klass), TYPE_MASTER, MasterClass))
                             ^
test.vala.c:248:11: note: in expansion of macro ‘MASTER_CLASS’
  _tmp0_ = MASTER_CLASS (child_parent_class)->get_prop (G_TYPE_CHECK_INSTANCE_CAST (self, TYPE_MASTER, Master));
           ^
test.vala.c: In function ‘child_real_set_prop’:
test.vala.c:259:103: error: incompatible type for argument 2 of ‘((MasterClass *)g_type_check_class_cast((GTypeClass *)child_parent_class,  master_get_type()))->set_prop’
  MASTER_CLASS (child_parent_class)->set_prop (G_TYPE_CHECK_INSTANCE_CAST (self, TYPE_MASTER, Master), _tmp0_);
                                                                                                       ^
test.vala.c:259:103: note: expected ‘Test * {aka struct _Test *}’ but argument is of type ‘Test {aka struct _Test}’
error: cc exited with status 256
Compilation failed: 1 error(s), 0 warning(s)

The attached patch fix the access of base property in overridden set when time was a boxed type