From 7a91aed7fea7952044ce6df1269f88900f99e013 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rgrossekunst@nvidia.com>
Date: Sun, 12 Oct 2025 10:50:25 -0700
Subject: [PATCH] [skip ci] Pin-pointing `decref in C++ dtor`

Based on manual review of:

$ cd include/pybind11/
$ git grep -e Py_DECREF -e Py_XDECREF | cut -d: -f1 | uniq
conduit/pybind11_conduit_v1.h
detail/class.h
detail/cpp_conduit.h
detail/type_caster_base.h
gil_safe_call_once.h
numpy.h
pybind11.h
pytypes.h
stl.h
stl/filesystem.h
trampoline_self_life_support.h
---
 include/pybind11/detail/type_caster_base.h      | 6 +++---
 include/pybind11/trampoline_self_life_support.h | 4 +++-
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/include/pybind11/detail/type_caster_base.h b/include/pybind11/detail/type_caster_base.h
index a2512a5527..073909b10b 100644
--- a/include/pybind11/detail/type_caster_base.h
+++ b/include/pybind11/detail/type_caster_base.h
@@ -77,7 +77,7 @@ class loader_life_support {
         }
         frame = parent;
         for (auto *item : keep_alive) {
-            Py_DECREF(item);
+            Py_DECREF(item); // decref in C++ dtor: always safe?
         }
     }
 
@@ -727,7 +727,7 @@ struct shared_ptr_parent_life_support {
     // NOLINTNEXTLINE(readability-make-member-function-const)
     void operator()(void *) {
         gil_scoped_acquire gil;
-        Py_DECREF(parent);
+        Py_DECREF(parent); // decref in C++ dtor: NEEDS can_decref_now
     }
 };
 
@@ -741,7 +741,7 @@ struct shared_ptr_trampoline_self_life_support {
     // NOLINTNEXTLINE(readability-make-member-function-const)
     void operator()(void *) {
         gil_scoped_acquire gil;
-        Py_DECREF(self);
+        Py_DECREF(self); // decref in C++ dtor: NEEDS can_decref_now
     }
 };
 
diff --git a/include/pybind11/trampoline_self_life_support.h b/include/pybind11/trampoline_self_life_support.h
index cbfec7f974..7715aa271f 100644
--- a/include/pybind11/trampoline_self_life_support.h
+++ b/include/pybind11/trampoline_self_life_support.h
@@ -42,7 +42,9 @@ struct trampoline_self_life_support {
                 v_h.value_ptr() = nullptr;
                 v_h.holder<smart_holder>().release_disowned();
                 detail::deregister_instance(v_h.inst, value_void_ptr, v_h.type);
-                Py_DECREF((PyObject *) v_h.inst); // Must be after deregister.
+                Py_DECREF(
+                    (PyObject *)
+                        v_h.inst); // Must be after deregister. // decref in C++ dtor: always safe?
                 PyGILState_Release(threadstate);
             }
         }