fix: test_factory_constructors.py failure triggered by test_register_duplicate_class (#2564)

* Demonstrate test_factory_constructors.py failure without functional changes from #2335

* Revert "Demonstrate test_factory_constructors.py failure without functional changes from #2335"

This reverts commit ca33a8021fc2a3617c3356b188796528f4594419.

* Fix test crash where registered Python type gets garbage collected

* Clean up some more internal structures when class objects go out of scope

* Reduce length of std::erase_if-in-C++20 comment

* Clean up code for cleaning up type internals

* Move cleaning up of type info in internals to tp_dealloc on pybind11_metaclass

include/pybind11/detail/class.h

@@ -193,6 +193,44 @@ extern "C" inline PyObject *pybind11_meta_call(PyObject *type, PyObject *args, P
     return self;
 }
 
+/// Cleanup the type-info for a pybind11-registered type.
+extern "C" inline void pybind11_meta_dealloc(PyObject *obj) {
+    auto *type = (PyTypeObject *) obj;
+    auto &internals = get_internals();
+
+    // A pybind11-registered type will:
+    // 1) be found in internals.registered_types_py
+    // 2) have exactly one associated `detail::type_info`
+    auto found_type = internals.registered_types_py.find(type);
+    if (found_type != internals.registered_types_py.end() &&
+        found_type->second.size() == 1 &&
+        found_type->second[0]->type == type) {
+
+        auto *tinfo = found_type->second[0];
+        auto tindex = std::type_index(*tinfo->cpptype);
+        internals.direct_conversions.erase(tindex);
+
+        if (tinfo->module_local)
+            registered_local_types_cpp().erase(tindex);
+        else
+            internals.registered_types_cpp.erase(tindex);
+        internals.registered_types_py.erase(tinfo->type);
+
+        // Actually just `std::erase_if`, but that's only available in C++20
+        auto &cache = internals.inactive_override_cache;
+        for (auto it = cache.begin(), last = cache.end(); it != last; ) {
+            if (it->first == (PyObject *) tinfo->type)
+                it = cache.erase(it);
+            else
+                ++it;
+        }
+
+        delete tinfo;
+    }
+
+    PyType_Type.tp_dealloc(obj);
+}
+
 /** This metaclass is assigned by default to all pybind11 types and is required in order
     for static properties to function correctly. Users may override this using `py::metaclass`.
     Return value: New reference. */
@@ -225,6 +263,8 @@ inline PyTypeObject* make_default_metaclass() {
     type->tp_getattro = pybind11_meta_getattro;
 #endif
 
+    type->tp_dealloc = pybind11_meta_dealloc;
+
     if (PyType_Ready(type) < 0)
         pybind11_fail("make_default_metaclass(): failure in PyType_Ready()!");
 

tests/test_class.py

@@ -434,7 +434,6 @@ def test_base_and_derived_nested_scope():
     assert m.DerivedWithNested.Nested.get_name() == "DerivedWithNested::Nested"
 
 
-@pytest.mark.skip("See https://github.com/pybind/pybind11/pull/2564")
 def test_register_duplicate_class():
     import types