Fix bindings of classes derived from a trampoline

Binding of deeper (>= 2 levels) class hierarchies that inherit from
a trampoline class fail in Python 3.12+. For example, in the added test
suite class ``SiameseCat``, this produces the following error message:

Critical nanobind error: nanobind::detail::nb_type_new("SiameseCat"): type construction failed: TypeError: tp_basicsize for type 'test_classes_ext.SiameseCat' (56) is too small for base 'test_classes_ext.Cat' (88)!

The trick to make Python happy is to carefully walk through the parent
classes to look for trampolines. Previously, only the direct parent was
checked.

Author: wjakob

src/nb_type.cpp

@@ -1206,11 +1206,28 @@ PyObject *nb_type_new(const type_init_data *t) noexcept {
 
         /* Handle a corner case (base class larger than derived class)
            which can arise when extending trampoline base classes */
-        size_t base_basicsize = sizeof(nb_inst) + tb->size;
-        if (tb->align > ptr_size)
-            base_basicsize += tb->align - ptr_size;
-        if (base_basicsize > basicsize)
-            basicsize = base_basicsize;
+
+        PyTypeObject *base_2 = (PyTypeObject *) base;
+        type_data *tb_2 = tb;
+
+        do {
+            size_t base_basicsize = sizeof(nb_inst) + tb_2->size;
+            if (tb_2->align > ptr_size)
+                base_basicsize += tb_2->align - ptr_size;
+            if (base_basicsize > basicsize)
+                basicsize = base_basicsize;
+
+#if !defined(Py_LIMITED_API)
+            base_2 = base_2->tp_base;
+#else
+            base_2 = (PyTypeObject *) PyType_GetSlot(base_2, Py_tp_base);
+#endif
+
+            if (!base_2 || !nb_type_check((PyObject *) base_2))
+                break;
+
+            tb_2 = nb_type_data(base_2);
+        } while (true);
     }
 
     bool base_intrusive_ptr =

tests/test_classes.cpp

@@ -261,6 +261,8 @@ NB_MODULE(test_classes_ext, m) {
         std::string s;
     };
 
+    struct SiameseCat : Cat { };
+
     struct Foo { };
 
     auto animal = nb::class_<Animal, PyAnimal>(m, "Animal")
@@ -274,6 +276,9 @@ NB_MODULE(test_classes_ext, m) {
     nb::class_<Cat>(m, "Cat", animal)
         .def(nb::init<const std::string &>());
 
+    nb::class_<SiameseCat, Cat> sc(m, "SiameseCat");
+    (void) sc;
+
     m.def("go", [](Animal *a) {
         return a->name() + " says " + a->what();
     });