Bring in tests/test_class_sh_mi_thunks.cpp,py from smart_holder branch as-is (Segmentation fault).

tests/test_class_sh_mi_thunks.cpp

@@ -0,0 +1,100 @@
+#include <pybind11/pybind11.h>
+#include <pybind11/smart_holder.h>
+
+#include "pybind11_tests.h"
+
+#include <cstddef>
+#include <memory>
+#include <vector>
+
+namespace test_class_sh_mi_thunks {
+
+// For general background: https://shaharmike.com/cpp/vtable-part2/
+// C++ vtables - Part 2 - Multiple Inheritance
+// ... the compiler creates a 'thunk' method that corrects `this` ...
+
+struct Base0 {
+    virtual ~Base0() = default;
+    Base0() = default;
+    Base0(const Base0 &) = delete;
+};
+
+struct Base1 {
+    virtual ~Base1() = default;
+    // Using `vector` here because it is known to make this test very sensitive to bugs.
+    std::vector<int> vec = {1, 2, 3, 4, 5};
+    Base1() = default;
+    Base1(const Base1 &) = delete;
+};
+
+struct Derived : Base1, Base0 {
+    ~Derived() override = default;
+    Derived() = default;
+    Derived(const Derived &) = delete;
+};
+
+} // namespace test_class_sh_mi_thunks
+
+PYBIND11_SMART_HOLDER_TYPE_CASTERS(test_class_sh_mi_thunks::Base0)
+PYBIND11_SMART_HOLDER_TYPE_CASTERS(test_class_sh_mi_thunks::Base1)
+PYBIND11_SMART_HOLDER_TYPE_CASTERS(test_class_sh_mi_thunks::Derived)
+
+TEST_SUBMODULE(class_sh_mi_thunks, m) {
+    using namespace test_class_sh_mi_thunks;
+
+    m.def("ptrdiff_drvd_base0", []() {
+        auto drvd = std::unique_ptr<Derived>(new Derived);
+        auto *base0 = dynamic_cast<Base0 *>(drvd.get());
+        return std::ptrdiff_t(reinterpret_cast<char *>(drvd.get())
+                              - reinterpret_cast<char *>(base0));
+    });
+
+    py::classh<Base0>(m, "Base0");
+    py::classh<Base1>(m, "Base1");
+    py::classh<Derived, Base1, Base0>(m, "Derived");
+
+    m.def(
+        "get_drvd_as_base0_raw_ptr",
+        []() {
+            auto *drvd = new Derived;
+            auto *base0 = dynamic_cast<Base0 *>(drvd);
+            return base0;
+        },
+        py::return_value_policy::take_ownership);
+
+    m.def("get_drvd_as_base0_shared_ptr", []() {
+        auto drvd = std::make_shared<Derived>();
+        auto base0 = std::dynamic_pointer_cast<Base0>(drvd);
+        return base0;
+    });
+
+    m.def("get_drvd_as_base0_unique_ptr", []() {
+        auto drvd = std::unique_ptr<Derived>(new Derived);
+        auto base0 = std::unique_ptr<Base0>(std::move(drvd));
+        return base0;
+    });
+
+    m.def("vec_size_base0_raw_ptr", [](const Base0 *obj) {
+        const auto *obj_der = dynamic_cast<const Derived *>(obj);
+        if (obj_der == nullptr) {
+            return std::size_t(0);
+        }
+        return obj_der->vec.size();
+    });
+
+    m.def("vec_size_base0_shared_ptr", [](const std::shared_ptr<Base0> &obj) -> std::size_t {
+        const auto obj_der = std::dynamic_pointer_cast<Derived>(obj);
+        if (!obj_der) {
+            return std::size_t(0);
+        }
+        return obj_der->vec.size();
+    });
+
+    m.def("vec_size_base0_unique_ptr", [](std::unique_ptr<Base0> obj) -> std::size_t {
+        const auto *obj_der = dynamic_cast<const Derived *>(obj.get());
+        if (obj_der == nullptr) {
+            return std::size_t(0);
+        }
+        return obj_der->vec.size();
+    });
+}

tests/test_class_sh_mi_thunks.py

@@ -0,0 +1,54 @@
+from __future__ import annotations
+
+import pytest
+
+from pybind11_tests import class_sh_mi_thunks as m
+
+
+def test_ptrdiff_drvd_base0():
+    ptrdiff = m.ptrdiff_drvd_base0()
+    # A failure here does not (necessarily) mean that there is a bug, but that
+    # test_class_sh_mi_thunks is not exercising what it is supposed to.
+    # If this ever fails on some platforms: use pytest.skip()
+    # If this ever fails on all platforms: don't know, seems extremely unlikely.
+    assert ptrdiff != 0
+
+
+@pytest.mark.parametrize(
+    "vec_size_fn",
+    [
+        m.vec_size_base0_raw_ptr,
+        m.vec_size_base0_shared_ptr,
+    ],
+)
+@pytest.mark.parametrize(
+    "get_fn",
+    [
+        m.get_drvd_as_base0_raw_ptr,
+        m.get_drvd_as_base0_shared_ptr,
+        m.get_drvd_as_base0_unique_ptr,
+    ],
+)
+def test_get_vec_size_raw_shared(get_fn, vec_size_fn):
+    obj = get_fn()
+    assert vec_size_fn(obj) == 5
+
+
+@pytest.mark.parametrize(
+    "get_fn", [m.get_drvd_as_base0_raw_ptr, m.get_drvd_as_base0_unique_ptr]
+)
+def test_get_vec_size_unique(get_fn):
+    obj = get_fn()
+    assert m.vec_size_base0_unique_ptr(obj) == 5
+    with pytest.raises(ValueError, match="Python instance was disowned"):
+        m.vec_size_base0_unique_ptr(obj)
+
+
+def test_get_shared_vec_size_unique():
+    obj = m.get_drvd_as_base0_shared_ptr()
+    with pytest.raises(ValueError) as exc_info:
+        m.vec_size_base0_unique_ptr(obj)
+    assert (
+        str(exc_info.value)
+        == "Cannot disown external shared_ptr (loaded_as_unique_ptr)."
+    )