diff --git a/guide/src/class.md b/guide/src/class.md
index 6b164ab1..cc5d4c94 100644
--- a/guide/src/class.md
+++ b/guide/src/class.md
@@ -757,7 +757,7 @@ impl pyo3::class::impl_::PyClassImpl for MyClass {
     type BaseType = PyAny;
     type ThreadChecker = pyo3::class::impl_::ThreadCheckerStub<MyClass>;
 
-    fn for_each_method_def(visitor: impl FnMut(&pyo3::class::PyMethodDefType)) {
+    fn for_each_method_def(visitor: &mut dyn FnMut(&pyo3::class::PyMethodDefType)) {
         use pyo3::class::impl_::*;
         let collector = PyClassImplCollector::<MyClass>::new();
         collector.py_methods().iter()
diff --git a/pyo3-macros-backend/src/pyclass.rs b/pyo3-macros-backend/src/pyclass.rs
index 4d568d79..669e07e6 100644
--- a/pyo3-macros-backend/src/pyclass.rs
+++ b/pyo3-macros-backend/src/pyclass.rs
@@ -436,7 +436,7 @@ fn impl_class(
             type BaseType = #base;
             type ThreadChecker = #thread_checker;
 
-            fn for_each_method_def(visitor: impl FnMut(&pyo3::class::PyMethodDefType)) {
+            fn for_each_method_def(visitor: &mut dyn FnMut(&pyo3::class::PyMethodDefType)) {
                 use pyo3::class::impl_::*;
                 let collector = PyClassImplCollector::<Self>::new();
                 #iter_py_methods
diff --git a/src/class/impl_.rs b/src/class/impl_.rs
index 3113be2b..169f59b3 100644
--- a/src/class/impl_.rs
+++ b/src/class/impl_.rs
@@ -65,7 +65,7 @@ pub trait PyClassImpl: Sized {
     ///    can be accessed by multiple threads by `threading` module.
     type ThreadChecker: PyClassThreadChecker<Self>;
 
-    fn for_each_method_def(_visitor: impl FnMut(&PyMethodDefType)) {}
+    fn for_each_method_def(_visitor: &mut dyn FnMut(&PyMethodDefType)) {}
     fn get_new() -> Option<ffi::newfunc> {
         None
     }
diff --git a/src/pyclass.rs b/src/pyclass.rs
index da2d3e2a..fb02e501 100644
--- a/src/pyclass.rs
+++ b/src/pyclass.rs
@@ -206,13 +206,13 @@ where
     }
 
     // normal methods
-    let methods = py_class_method_defs::<T>();
+    let methods = py_class_method_defs(&T::for_each_method_def);
     if !methods.is_empty() {
         slots.push(ffi::Py_tp_methods, into_raw(methods));
     }
 
     // properties
-    let props = py_class_properties::<T>();
+    let props = py_class_properties(T::Dict::IS_DUMMY, &T::for_each_method_def);
     if !props.is_empty() {
         slots.push(ffi::Py_tp_getset, into_raw(props));
     }
@@ -230,7 +230,7 @@ where
         name: get_type_name::<T>(module_name)?,
         basicsize: std::mem::size_of::<T::Layout>() as c_int,
         itemsize: 0,
-        flags: py_class_flags::<T>(has_gc_methods),
+        flags: py_class_flags(has_gc_methods, T::IS_GC, T::IS_BASETYPE),
         slots: slots.0.as_mut_ptr(),
     };
 
@@ -299,22 +299,24 @@ fn tp_init_additional<T: PyClass>(type_object: *mut ffi::PyTypeObject) {
 #[cfg(any(Py_LIMITED_API, Py_3_10))]
 fn tp_init_additional<T: PyClass>(_type_object: *mut ffi::PyTypeObject) {}
 
-fn py_class_flags<T: PyClass + PyTypeInfo>(has_gc_methods: bool) -> c_uint {
-    let mut flags = if has_gc_methods || T::IS_GC {
+fn py_class_flags(has_gc_methods: bool, is_gc: bool, is_basetype: bool) -> c_uint {
+    let mut flags = if has_gc_methods || is_gc {
         ffi::Py_TPFLAGS_DEFAULT | ffi::Py_TPFLAGS_HAVE_GC
     } else {
         ffi::Py_TPFLAGS_DEFAULT
     };
-    if T::IS_BASETYPE {
+    if is_basetype {
         flags |= ffi::Py_TPFLAGS_BASETYPE;
     }
     flags.try_into().unwrap()
 }
 
-fn py_class_method_defs<T: PyClassImpl>() -> Vec<ffi::PyMethodDef> {
+fn py_class_method_defs(
+    for_each_method_def: &dyn Fn(&mut dyn FnMut(&PyMethodDefType)),
+) -> Vec<ffi::PyMethodDef> {
     let mut defs = Vec::new();
 
-    T::for_each_method_def(|def| match def {
+    for_each_method_def(&mut |def| match def {
         PyMethodDefType::Method(def)
         | PyMethodDefType::Class(def)
         | PyMethodDefType::Static(def) => {
@@ -381,10 +383,13 @@ const PY_GET_SET_DEF_INIT: ffi::PyGetSetDef = ffi::PyGetSetDef {
 };
 
 #[allow(clippy::clippy::collapsible_if)] // for if cfg!
-fn py_class_properties<T: PyClass>() -> Vec<ffi::PyGetSetDef> {
+fn py_class_properties(
+    is_dummy: bool,
+    for_each_method_def: &dyn Fn(&mut dyn FnMut(&PyMethodDefType)),
+) -> Vec<ffi::PyGetSetDef> {
     let mut defs = std::collections::HashMap::new();
 
-    T::for_each_method_def(|def| match def {
+    for_each_method_def(&mut |def| match def {
         PyMethodDefType::Getter(getter) => {
             getter.copy_to(defs.entry(getter.name).or_insert(PY_GET_SET_DEF_INIT));
         }
@@ -398,7 +403,7 @@ fn py_class_properties<T: PyClass>() -> Vec<ffi::PyGetSetDef> {
 
     // PyPy doesn't automatically adds __dict__ getter / setter.
     // PyObject_GenericGetDict not in the limited API until Python 3.10.
-    push_dict_getset::<T>(&mut props);
+    push_dict_getset(&mut props, is_dummy);
 
     if !props.is_empty() {
         props.push(unsafe { std::mem::zeroed() });
@@ -407,8 +412,8 @@ fn py_class_properties<T: PyClass>() -> Vec<ffi::PyGetSetDef> {
 }
 
 #[cfg(not(any(PyPy, all(Py_LIMITED_API, not(Py_3_10)))))]
-fn push_dict_getset<T: PyClass>(props: &mut Vec<ffi::PyGetSetDef>) {
-    if !T::Dict::IS_DUMMY {
+fn push_dict_getset(props: &mut Vec<ffi::PyGetSetDef>, is_dummy: bool) {
+    if !is_dummy {
         props.push(ffi::PyGetSetDef {
             name: "__dict__\0".as_ptr() as *mut c_char,
             get: Some(ffi::PyObject_GenericGetDict),
@@ -420,4 +425,4 @@ fn push_dict_getset<T: PyClass>(props: &mut Vec<ffi::PyGetSetDef>) {
 }
 
 #[cfg(any(PyPy, all(Py_LIMITED_API, not(Py_3_10))))]
-fn push_dict_getset<T: PyClass>(_: &mut Vec<ffi::PyGetSetDef>) {}
+fn push_dict_getset(_: &mut Vec<ffi::PyGetSetDef>, _is_dummy: bool) {}
diff --git a/src/type_object.rs b/src/type_object.rs
index ebf7df8a..cbc9e326 100644
--- a/src/type_object.rs
+++ b/src/type_object.rs
@@ -104,6 +104,17 @@ impl LazyStaticType {
             })
         });
 
+        self.ensure_init(py, type_object, T::NAME, &T::for_each_method_def);
+        type_object
+    }
+
+    fn ensure_init(
+        &self,
+        py: Python,
+        type_object: *mut ffi::PyTypeObject,
+        name: &str,
+        for_each_method_def: &dyn Fn(&mut dyn FnMut(&PyMethodDefType)),
+    ) {
         // We might want to fill the `tp_dict` with python instances of `T`
         // itself. In order to do so, we must first initialize the type object
         // with an empty `tp_dict`: now we can create instances of `T`.
@@ -117,7 +128,7 @@ impl LazyStaticType {
 
         if self.tp_dict_filled.get(py).is_some() {
             // `tp_dict` is already filled: ok.
-            return type_object;
+            return;
         }
 
         {
@@ -126,7 +137,7 @@ impl LazyStaticType {
             if threads.contains(&thread_id) {
                 // Reentrant call: just return the type object, even if the
                 // `tp_dict` is not filled yet.
-                return type_object;
+                return;
             }
             threads.push(thread_id);
         }
@@ -136,7 +147,7 @@ impl LazyStaticType {
         // means that another thread can continue the initialization in the
         // meantime: at worst, we'll just make a useless computation.
         let mut items = vec![];
-        T::for_each_method_def(|def| {
+        for_each_method_def(&mut |def| {
             if let PyMethodDefType::ClassAttribute(attr) = def {
                 items.push((
                     extract_cstr_or_leak_cstring(
@@ -162,10 +173,8 @@ impl LazyStaticType {
 
         if let Err(err) = result {
             err.clone_ref(py).print(py);
-            panic!("An error occured while initializing `{}.__dict__`", T::NAME);
+            panic!("An error occured while initializing `{}.__dict__`", name);
         }
-
-        type_object
     }
 }