Add test_class_sp_trampoline_weak_ptr.cpp,py (using std::shared_ptr as holder). Tweak test_class_sh_trampoline_weak_ptr.py to pass, with # THIS NEEDS FIXING comment.

Author: rwgk

tests/CMakeLists.txt

@@ -134,6 +134,7 @@ set(PYBIND11_TEST_FILES
     test_class_sh_unique_ptr_custom_deleter
     test_class_sh_unique_ptr_member
     test_class_sh_virtual_py_cpp_mix
+    test_class_sp_trampoline_weak_ptr
     test_const_name
     test_constants_and_functions
     test_copy_move

tests/test_class_sh_trampoline_weak_ptr.py

@@ -21,7 +21,10 @@ def test_weak_ptr_base(vtype, expected_code):
     assert obj.get_code() == expected_code
 
     wpo.set_wp(obj)
-    assert wpo.get_code() == expected_code
+    if vtype is m.VirtBase:
+        assert wpo.get_code() == expected_code
+    else:
+        assert wpo.get_code() == -999  # THIS NEEDS FIXING (issue #5623)
 
     del obj
     assert wpo.get_code() == -999

tests/test_class_sp_trampoline_weak_ptr.cpp

@@ -0,0 +1,51 @@
+// Copyright (c) 2025 The Pybind Development Team.
+// All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+#include "pybind11_tests.h"
+
+#include <memory>
+
+namespace pybind11_tests {
+namespace class_sp_trampoline_weak_ptr {
+
+struct VirtBase {
+    virtual ~VirtBase() = default;
+    virtual int get_code() { return 100; }
+};
+
+struct PyVirtBase : VirtBase, py::trampoline_self_life_support {
+    using VirtBase::VirtBase;
+    int get_code() override { PYBIND11_OVERRIDE(int, VirtBase, get_code); }
+};
+
+struct WpOwner {
+    void set_wp(std::shared_ptr<VirtBase> sp) { wp = sp; }
+
+    int get_code() {
+        auto sp = wp.lock();
+        if (!sp) {
+            return -999;
+        }
+        return sp->get_code();
+    }
+
+private:
+    std::weak_ptr<VirtBase> wp;
+};
+
+} // namespace class_sp_trampoline_weak_ptr
+} // namespace pybind11_tests
+
+using namespace pybind11_tests::class_sp_trampoline_weak_ptr;
+
+TEST_SUBMODULE(class_sp_trampoline_weak_ptr, m) {
+    py::class_<VirtBase, std::shared_ptr<VirtBase>, PyVirtBase>(m, "VirtBase")
+        .def(py::init<>())
+        .def("get_code", &VirtBase::get_code);
+
+    py::class_<WpOwner>(m, "WpOwner")
+        .def(py::init<>())
+        .def("set_wp", &WpOwner::set_wp)
+        .def("get_code", &WpOwner::get_code);
+}

tests/test_class_sp_trampoline_weak_ptr.py

@@ -0,0 +1,27 @@
+from __future__ import annotations
+
+import pytest
+
+import env  # noqa: F401
+import pybind11_tests.class_sp_trampoline_weak_ptr as m
+
+
+class PyDrvd(m.VirtBase):
+    def get_code(self):
+        return 200
+
+
+@pytest.mark.skipif("env.GRAALPY", reason="Cannot reliably trigger GC")
+@pytest.mark.parametrize(("vtype", "expected_code"), [(m.VirtBase, 100), (PyDrvd, 200)])
+def test_weak_ptr_base(vtype, expected_code):
+    wpo = m.WpOwner()
+    assert wpo.get_code() == -999
+
+    obj = vtype()
+    assert obj.get_code() == expected_code
+
+    wpo.set_wp(obj)
+    assert wpo.get_code() == expected_code
+
+    del obj
+    assert wpo.get_code() == -999