diff --git a/python/BUILD.bazel b/python/BUILD.bazel
index f604bad..7f2b247 100644
--- a/python/BUILD.bazel
+++ b/python/BUILD.bazel
@@ -190,6 +190,7 @@
     ],
     target_compatible_with = select(_message_target_compatible_with),
     deps = [
+        "//upb:base",
         "//upb:descriptor_upb_proto_reflection",
         "//upb:eps_copy_input_stream",
         "//upb:hash",
diff --git a/python/descriptor.c b/python/descriptor.c
index ea4c028..c5af6dd 100644
--- a/python/descriptor.c
+++ b/python/descriptor.c
@@ -12,6 +12,7 @@
 #include "python/descriptor_pool.h"
 #include "python/message.h"
 #include "python/protobuf.h"
+#include "upb/base/upcast.h"
 #include "upb/reflection/def.h"
 #include "upb/util/def_to_proto.h"
 
@@ -391,7 +392,7 @@
 static PyObject* PyUpb_Descriptor_GetOptions(PyObject* _self, PyObject* args) {
   PyUpb_DescriptorBase* self = (void*)_self;
   return PyUpb_DescriptorBase_GetOptions(
-      &self->options, upb_MessageDef_Options(self->def),
+      &self->options, UPB_UPCAST(upb_MessageDef_Options(self->def)),
       &google__protobuf__MessageOptions_msg_init,
       PYUPB_DESCRIPTOR_PROTO_PACKAGE ".MessageOptions");
 }
@@ -399,7 +400,7 @@
 static PyObject* PyUpb_Descriptor_GetFeatures(PyObject* _self, PyObject* args) {
   PyUpb_DescriptorBase* self = (void*)_self;
   return PyUpb_DescriptorBase_GetFeatures(
-      &self->features, upb_MessageDef_ResolvedFeatures(self->def));
+      &self->features, UPB_UPCAST(upb_MessageDef_ResolvedFeatures(self->def)));
 }
 
 static PyObject* PyUpb_Descriptor_CopyToProto(PyObject* _self,
@@ -700,10 +701,10 @@
     {0, NULL}};
 
 static PyType_Spec PyUpb_Descriptor_Spec = {
-    PYUPB_MODULE_NAME ".Descriptor",  // tp_name
-    sizeof(PyUpb_DescriptorBase),     // tp_basicsize
-    0,                                // tp_itemsize
-    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC, // tp_flags
+    PYUPB_MODULE_NAME ".Descriptor",          // tp_name
+    sizeof(PyUpb_DescriptorBase),             // tp_basicsize
+    0,                                        // tp_itemsize
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,  // tp_flags
     PyUpb_Descriptor_Slots,
 };
 
@@ -809,7 +810,7 @@
                                                  PyObject* args) {
   PyUpb_DescriptorBase* self = (void*)_self;
   return PyUpb_DescriptorBase_GetOptions(
-      &self->options, upb_EnumDef_Options(self->def),
+      &self->options, UPB_UPCAST(upb_EnumDef_Options(self->def)),
       &google__protobuf__EnumOptions_msg_init,
       PYUPB_DESCRIPTOR_PROTO_PACKAGE ".EnumOptions");
 }
@@ -818,7 +819,7 @@
                                                   PyObject* args) {
   PyUpb_DescriptorBase* self = (void*)_self;
   return PyUpb_DescriptorBase_GetFeatures(
-      &self->features, upb_EnumDef_ResolvedFeatures(self->def));
+      &self->features, UPB_UPCAST(upb_EnumDef_ResolvedFeatures(self->def)));
 }
 
 static PyObject* PyUpb_EnumDescriptor_CopyToProto(PyObject* _self,
@@ -908,7 +909,7 @@
                                                       PyObject* args) {
   PyUpb_DescriptorBase* self = (void*)_self;
   return PyUpb_DescriptorBase_GetOptions(
-      &self->options, upb_EnumValueDef_Options(self->def),
+      &self->options, UPB_UPCAST(upb_EnumValueDef_Options(self->def)),
       &google__protobuf__EnumValueOptions_msg_init,
       PYUPB_DESCRIPTOR_PROTO_PACKAGE ".EnumValueOptions");
 }
@@ -917,7 +918,8 @@
                                                        PyObject* args) {
   PyUpb_DescriptorBase* self = (void*)_self;
   return PyUpb_DescriptorBase_GetFeatures(
-      &self->features, upb_EnumValueDef_ResolvedFeatures(self->def));
+      &self->features,
+      UPB_UPCAST(upb_EnumValueDef_ResolvedFeatures(self->def)));
 }
 
 static PyGetSetDef PyUpb_EnumValueDescriptor_Getters[] = {
@@ -1121,7 +1123,7 @@
                                                   PyObject* args) {
   PyUpb_DescriptorBase* self = (void*)_self;
   return PyUpb_DescriptorBase_GetOptions(
-      &self->options, upb_FieldDef_Options(self->def),
+      &self->options, UPB_UPCAST(upb_FieldDef_Options(self->def)),
       &google__protobuf__FieldOptions_msg_init,
       PYUPB_DESCRIPTOR_PROTO_PACKAGE ".FieldOptions");
 }
@@ -1130,7 +1132,7 @@
                                                    PyObject* args) {
   PyUpb_DescriptorBase* self = (void*)_self;
   return PyUpb_DescriptorBase_GetFeatures(
-      &self->features, upb_FieldDef_ResolvedFeatures(self->def));
+      &self->features, UPB_UPCAST(upb_FieldDef_ResolvedFeatures(self->def)));
 }
 
 static PyGetSetDef PyUpb_FieldDescriptor_Getters[] = {
@@ -1381,7 +1383,7 @@
                                                  PyObject* args) {
   PyUpb_DescriptorBase* self = (void*)_self;
   return PyUpb_DescriptorBase_GetOptions(
-      &self->options, upb_FileDef_Options(self->def),
+      &self->options, UPB_UPCAST(upb_FileDef_Options(self->def)),
       &google__protobuf__FileOptions_msg_init,
       PYUPB_DESCRIPTOR_PROTO_PACKAGE ".FileOptions");
 }
@@ -1390,7 +1392,7 @@
                                                   PyObject* args) {
   PyUpb_DescriptorBase* self = (void*)_self;
   return PyUpb_DescriptorBase_GetFeatures(
-      &self->features, upb_FileDef_ResolvedFeatures(self->def));
+      &self->features, UPB_UPCAST(upb_FileDef_ResolvedFeatures(self->def)));
 }
 
 static PyObject* PyUpb_FileDescriptor_CopyToProto(PyObject* _self,
@@ -1515,7 +1517,7 @@
                                                    PyObject* args) {
   PyUpb_DescriptorBase* self = (void*)_self;
   return PyUpb_DescriptorBase_GetOptions(
-      &self->options, upb_MethodDef_Options(self->def),
+      &self->options, UPB_UPCAST(upb_MethodDef_Options(self->def)),
       &google__protobuf__MethodOptions_msg_init,
       PYUPB_DESCRIPTOR_PROTO_PACKAGE ".MethodOptions");
 }
@@ -1524,7 +1526,7 @@
                                                     PyObject* args) {
   PyUpb_DescriptorBase* self = (void*)_self;
   return PyUpb_DescriptorBase_GetFeatures(
-      &self->features, upb_MethodDef_ResolvedFeatures(self->def));
+      &self->features, UPB_UPCAST(upb_MethodDef_ResolvedFeatures(self->def)));
 }
 
 static PyObject* PyUpb_MethodDescriptor_CopyToProto(PyObject* _self,
@@ -1632,7 +1634,7 @@
                                                   PyObject* args) {
   PyUpb_DescriptorBase* self = (void*)_self;
   return PyUpb_DescriptorBase_GetOptions(
-      &self->options, upb_OneofDef_Options(self->def),
+      &self->options, UPB_UPCAST(upb_OneofDef_Options(self->def)),
       &google__protobuf__OneofOptions_msg_init,
       PYUPB_DESCRIPTOR_PROTO_PACKAGE ".OneofOptions");
 }
@@ -1641,7 +1643,7 @@
                                                    PyObject* args) {
   PyUpb_DescriptorBase* self = (void*)_self;
   return PyUpb_DescriptorBase_GetFeatures(
-      &self->features, upb_OneofDef_ResolvedFeatures(self->def));
+      &self->features, UPB_UPCAST(upb_OneofDef_ResolvedFeatures(self->def)));
 }
 
 static PyGetSetDef PyUpb_OneofDescriptor_Getters[] = {
@@ -1742,7 +1744,7 @@
                                                     PyObject* args) {
   PyUpb_DescriptorBase* self = (void*)_self;
   return PyUpb_DescriptorBase_GetOptions(
-      &self->options, upb_ServiceDef_Options(self->def),
+      &self->options, UPB_UPCAST(upb_ServiceDef_Options(self->def)),
       &google__protobuf__ServiceOptions_msg_init,
       PYUPB_DESCRIPTOR_PROTO_PACKAGE ".ServiceOptions");
 }
@@ -1751,7 +1753,7 @@
                                                      PyObject* args) {
   PyUpb_DescriptorBase* self = (void*)_self;
   return PyUpb_DescriptorBase_GetFeatures(
-      &self->features, upb_ServiceDef_ResolvedFeatures(self->def));
+      &self->features, UPB_UPCAST(upb_ServiceDef_ResolvedFeatures(self->def)));
 }
 
 static PyObject* PyUpb_ServiceDescriptor_CopyToProto(PyObject* _self,
diff --git a/python/descriptor_pool.c b/python/descriptor_pool.c
index 09efbda..089d31b 100644
--- a/python/descriptor_pool.c
+++ b/python/descriptor_pool.c
@@ -12,6 +12,7 @@
 #include "python/descriptor.h"
 #include "python/message.h"
 #include "python/protobuf.h"
+#include "upb/base/upcast.h"
 #include "upb/reflection/def.h"
 #include "upb/util/def_to_proto.h"
 
@@ -147,8 +148,7 @@
 static bool PyUpb_DescriptorPool_LoadDependentFiles(
     PyUpb_DescriptorPool* self, google_protobuf_FileDescriptorProto* proto) {
   size_t n;
-  const upb_StringView* deps =
-      google_protobuf_FileDescriptorProto_dependency(proto, &n);
+  const upb_StringView* deps = google_protobuf_FileDescriptorProto_dependency(proto, &n);
   for (size_t i = 0; i < n; i++) {
     const upb_FileDef* dep = upb_DefPool_FindFileByNameWithSize(
         self->symtab, deps[i].data, deps[i].size);
@@ -191,14 +191,13 @@
   if (file) {
     // If the existing file is equal to the new file, then silently ignore the
     // duplicate add.
-    google_protobuf_FileDescriptorProto* existing =
-        upb_FileDef_ToProto(file, arena);
+    google_protobuf_FileDescriptorProto* existing = upb_FileDef_ToProto(file, arena);
     if (!existing) {
       PyErr_SetNone(PyExc_MemoryError);
       goto done;
     }
     const upb_MessageDef* m = PyUpb_DescriptorPool_GetFileProtoDef();
-    if (upb_Message_IsEqual(proto, existing, m)) {
+    if (upb_Message_IsEqual(UPB_UPCAST(proto), UPB_UPCAST(existing), m)) {
       result = PyUpb_FileDescriptor_Get(file);
       goto done;
     }
diff --git a/python/map.c b/python/map.c
index 4330aae..bbbd636 100644
--- a/python/map.c
+++ b/python/map.c
@@ -184,7 +184,7 @@
     map = PyUpb_MapContainer_EnsureReified(_self);
     upb_Arena* arena = PyUpb_Arena_Get(self->arena);
     if (upb_FieldDef_IsSubMessage(val_f)) {
-      const upb_Message* m = upb_FieldDef_MessageSubDef(val_f);
+      const upb_MessageDef* m = upb_FieldDef_MessageSubDef(val_f);
       const upb_MiniTable* layout = upb_MessageDef_MiniTable(m);
       u_val.msg_val = upb_Message_New(layout, arena);
     } else {