Modify py::trampoline_self_life_support semantics: if trampoline class does not inherit from this class, preserve established Inheritance Slicing behavior.

rwgk reached this point with the help of ChatGPT:

* https://chatgpt.com/share/68056498-7d94-8008-8ff0-232e2aba451c

The only production code change in this commit is:

```
diff --git a/include/pybind11/detail/type_caster_base.h b/include/pybind11/detail/type_caster_base.h
index d4f9a41..f3d4530 100644
--- a/include/pybind11/detail/type_caster_base.h
+++ b/include/pybind11/detail/type_caster_base.h
@@ -776,6 +776,14 @@ struct load_helper : value_and_holder_helper {
                 if (released_ptr) {
                     return std::shared_ptr<T>(released_ptr, type_raw_ptr);
                 }
+                auto *self_life_support
+                    = dynamic_raw_ptr_cast_if_possible<trampoline_self_life_support>(type_raw_ptr);
+                if (self_life_support == nullptr) {
+                    std::shared_ptr<void> void_shd_ptr = hld.template as_shared_ptr<void>();
+                    std::shared_ptr<T> to_be_released(void_shd_ptr, type_raw_ptr);
+                    vptr_gd_ptr->released_ptr = to_be_released;
+                    return to_be_released;
+                }
                 std::shared_ptr<T> to_be_released(
                     type_raw_ptr, shared_ptr_trampoline_self_life_support(loaded_v_h.inst));
                 vptr_gd_ptr->released_ptr = to_be_released;
```

Author: rwgk

include/pybind11/detail/type_caster_base.h

@@ -776,6 +776,14 @@ struct load_helper : value_and_holder_helper {
                 if (released_ptr) {
                     return std::shared_ptr<T>(released_ptr, type_raw_ptr);
                 }
+                auto *self_life_support
+                    = dynamic_raw_ptr_cast_if_possible<trampoline_self_life_support>(type_raw_ptr);
+                if (self_life_support == nullptr) {
+                    std::shared_ptr<void> void_shd_ptr = hld.template as_shared_ptr<void>();
+                    std::shared_ptr<T> to_be_released(void_shd_ptr, type_raw_ptr);
+                    vptr_gd_ptr->released_ptr = to_be_released;
+                    return to_be_released;
+                }
                 std::shared_ptr<T> to_be_released(
                     type_raw_ptr, shared_ptr_trampoline_self_life_support(loaded_v_h.inst));
                 vptr_gd_ptr->released_ptr = to_be_released;

include/pybind11/pybind11.h

@@ -3416,14 +3416,23 @@ PYBIND11_NAMESPACE_END(detail)
 template <class T>
 function get_override(const T *this_ptr, const char *name) {
     auto *tinfo = detail::get_type_info(typeid(T));
+    fflush(stderr);
+    printf("\nLOOOK get_override tinfo truthy = %s\n", tinfo ? "YES" : "NO");
+    fflush(stdout);
     return tinfo ? detail::get_type_override(this_ptr, tinfo, name) : function();
 }
 
 #define PYBIND11_OVERRIDE_IMPL(ret_type, cname, name, ...)                                        \
     do {                                                                                          \
         pybind11::gil_scoped_acquire gil;                                                         \
+        fflush(stderr);                                                                           \
+        printf("\nLOOOK BEFORE static_cast<const cname *>(this)\n");                              \
+        fflush(stdout);                                                                           \
         pybind11::function override                                                               \
             = pybind11::get_override(static_cast<const cname *>(this), name);                     \
+        fflush(stderr);                                                                           \
+        printf("\nLOOOK  AFTER static_cast<const cname *>(this)\n");                              \
+        fflush(stdout);                                                                           \
         if (override) {                                                                           \
             auto o = override(__VA_ARGS__);                                                       \
             PYBIND11_WARNING_PUSH                                                                 \

tests/test_class_sh_trampoline_shared_ptr_cpp_arg.cpp

@@ -28,7 +28,7 @@ struct SpBase {
 
 std::shared_ptr<SpBase> pass_through_shd_ptr(const std::shared_ptr<SpBase> &obj) { return obj; }
 
-struct PySpBase : SpBase {
+struct PySpBase : SpBase, py::trampoline_self_life_support {
     using SpBase::SpBase;
     bool is_base_used() override { PYBIND11_OVERRIDE(bool, SpBase, is_base_used); }
 };

tests/test_class_sh_trampoline_weak_ptr.cpp

@@ -14,9 +14,15 @@ struct VirtBase {
     virtual int get_code() { return 100; }
 };
 
-struct PyVirtBase : VirtBase, py::trampoline_self_life_support {
+struct PyVirtBase : VirtBase /*, py::trampoline_self_life_support */ {
     using VirtBase::VirtBase;
     int get_code() override { PYBIND11_OVERRIDE(int, VirtBase, get_code); }
+
+    ~PyVirtBase() override {
+        fflush(stderr);
+        printf("\nLOOOK ~PyVirtBase()\n");
+        fflush(stdout);
+    }
 };
 
 struct WpOwner {
@@ -34,6 +40,10 @@ struct WpOwner {
     std::weak_ptr<VirtBase> wp;
 };
 
+std::shared_ptr<VirtBase> pass_through_sp_VirtBase(const std::shared_ptr<VirtBase> &sp) {
+    return sp;
+}
+
 } // namespace class_sh_trampoline_weak_ptr
 } // namespace pybind11_tests
 
@@ -48,4 +58,6 @@ TEST_SUBMODULE(class_sh_trampoline_weak_ptr, m) {
         .def(py::init<>())
         .def("set_wp", &WpOwner::set_wp)
         .def("get_code", &WpOwner::get_code);
+
+    m.def("pass_through_sp_VirtBase", pass_through_sp_VirtBase);
 }

tests/test_class_sh_trampoline_weak_ptr.py

@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+import gc
+
 import pytest
 
 import env
@@ -20,12 +22,22 @@ def test_weak_ptr_owner(vtype, expected_code):
     assert obj.get_code() == expected_code
 
     wpo.set_wp(obj)
-    if vtype is m.VirtBase:
-        assert wpo.get_code() == expected_code
-    else:
-        assert wpo.get_code() == -999  # THIS NEEDS FIXING (issue #5623)
+    assert wpo.get_code() == expected_code
 
     del obj
     if env.PYPY or env.GRAALPY:
         pytest.skip("Cannot reliably trigger GC")
     assert wpo.get_code() == -999
+
+
+@pytest.mark.parametrize(("vtype", "expected_code"), [(m.VirtBase, 100), (PyDrvd, 200)])
+def test_pass_through_sp_VirtBase(vtype, expected_code):
+    obj = vtype()
+    ptr = m.pass_through_sp_VirtBase(obj)
+    print("\nLOOOK BEFORE del obj", flush=True)
+    del obj
+    print("\nLOOOK  AFTER del obj", flush=True)
+    gc.collect()
+    print("\nLOOOK  AFTER gc.collect()", flush=True)
+    assert ptr.get_code() == expected_code
+    print("\nLOOOK  AFTER ptr.get_code()", flush=True)

tests/test_class_sp_trampoline_weak_ptr.cpp

@@ -17,6 +17,12 @@ struct VirtBase {
 struct PyVirtBase : VirtBase, py::trampoline_self_life_support {
     using VirtBase::VirtBase;
     int get_code() override { PYBIND11_OVERRIDE(int, VirtBase, get_code); }
+
+    ~PyVirtBase() override {
+        fflush(stderr);
+        printf("\nLOOOK ~PyVirtBase()\n");
+        fflush(stdout);
+    }
 };
 
 struct WpOwner {
@@ -48,6 +54,10 @@ struct SpOwner {
     std::shared_ptr<VirtBase> sp;
 };
 
+std::shared_ptr<VirtBase> pass_through_sp_VirtBase(const std::shared_ptr<VirtBase> &sp) {
+    return sp;
+}
+
 } // namespace class_sp_trampoline_weak_ptr
 } // namespace pybind11_tests
 
@@ -67,4 +77,6 @@ TEST_SUBMODULE(class_sp_trampoline_weak_ptr, m) {
         .def(py::init<>())
         .def("set_sp", &SpOwner::set_sp)
         .def("get_code", &SpOwner::get_code);
+
+    m.def("pass_through_sp_VirtBase", pass_through_sp_VirtBase);
 }

tests/test_class_sp_trampoline_weak_ptr.py

@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+import gc
+
 import pytest
 
 import env
@@ -42,7 +44,9 @@ def test_with_sp_owner(vtype, expected_code):
     del obj
     if env.PYPY or env.GRAALPY:
         pytest.skip("Cannot reliably trigger GC")
+    print("\nLOOOK BEFORE spo.get_code() AFTER del obj", flush=True)
     assert spo.get_code() == 100  # Inheritance slicing (issue #1333)
+    print("\nLOOOK  AFTER spo.get_code() AFTER del obj", flush=True)
 
 
 @pytest.mark.parametrize(("vtype", "expected_code"), [(m.VirtBase, 100), (PyDrvd, 200)])
@@ -67,3 +71,16 @@ def test_with_sp_and_wp_owners(vtype, expected_code):
 
     del spo
     assert wpo.get_code() == -999
+
+
+@pytest.mark.parametrize(("vtype", "expected_code"), [(m.VirtBase, 100), (PyDrvd, 200)])
+def test_pass_through_sp_VirtBase(vtype, expected_code):
+    obj = vtype()
+    ptr = m.pass_through_sp_VirtBase(obj)
+    print("\nLOOOK BEFORE del obj", flush=True)
+    del obj
+    print("\nLOOOK  AFTER del obj", flush=True)
+    gc.collect()
+    print("\nLOOOK  AFTER gc.collect()", flush=True)
+    assert ptr.get_code() == expected_code
+    print("\nLOOOK  AFTER ptr.get_code()", flush=True)