Copy test_ccccccccccccccccccccc.cpp,py from bakein branch as-is.

tests/CMakeLists.txt

@@ -117,6 +117,7 @@ set(PYBIND11_TEST_FILES
     test_builtin_casters
     test_call_policies
     test_callbacks
+    test_ccccccccccccccccccccc
     test_chrono
     test_class
     test_class_sh_basic

tests/test_ccccccccccccccccccccc.cpp

@@ -0,0 +1,104 @@
+#include <pybind11/smart_holder.h>
+
+#include "pybind11_tests.h"
+
+#include <memory>
+
+namespace pybind11_tests {
+namespace ccccccccccccccccccccc {
+
+// Diamond inheritance (copied from test_multiple_inheritance.cpp).
+struct B {
+    int val_b = 10;
+    B() = default;
+    B(const B &) = default;
+    virtual ~B() = default;
+};
+
+struct C0 : public virtual B {
+    int val_c0 = 20;
+};
+
+struct C1 : public virtual B {
+    int val_c1 = 21;
+};
+
+struct D : public C0, public C1 {
+    int val_d = 30;
+};
+
+void disown_b(std::unique_ptr<B>) {}
+
+const std::string fooNames[] = {"ShPtr_"};
+
+template <int SerNo>
+struct Foo {
+    std::string history;
+    explicit Foo(const std::string &history_) : history(history_) {}
+    Foo(const Foo &other) : history(other.history + "_CpCtor") {}
+    Foo(Foo &&other) noexcept : history(other.history + "_MvCtor") {}
+    Foo &operator=(const Foo &other) {
+        history = other.history + "_OpEqLv";
+        return *this;
+    }
+    Foo &operator=(Foo &&other) noexcept {
+        history = other.history + "_OpEqRv";
+        return *this;
+    }
+    std::string get_history() const { return "Foo" + fooNames[SerNo] + history; }
+};
+
+using FooShPtr = Foo<0>;
+
+} // namespace ccccccccccccccccccccc
+} // namespace pybind11_tests
+
+PYBIND11_SMART_HOLDER_TYPE_CASTERS(pybind11_tests::ccccccccccccccccccccc::B)
+PYBIND11_SMART_HOLDER_TYPE_CASTERS(pybind11_tests::ccccccccccccccccccccc::C0)
+PYBIND11_SMART_HOLDER_TYPE_CASTERS(pybind11_tests::ccccccccccccccccccccc::C1)
+PYBIND11_SMART_HOLDER_TYPE_CASTERS(pybind11_tests::ccccccccccccccccccccc::D)
+
+PYBIND11_TYPE_CASTER_BASE_HOLDER(pybind11_tests::ccccccccccccccccccccc::FooShPtr,
+                                 std::shared_ptr<pybind11_tests::ccccccccccccccccccccc::FooShPtr>)
+
+TEST_SUBMODULE(ccccccccccccccccccccc, m) {
+    using namespace pybind11_tests::ccccccccccccccccccccc;
+
+    py::class_<FooShPtr, std::shared_ptr<FooShPtr>>(m, "FooShPtr")
+        .def("get_history", &FooShPtr::get_history);
+
+    m.def("test_ShPtr_copy", []() {
+        auto o = std::make_shared<FooShPtr>("copy");
+        auto l = py::list();
+        l.append(o);
+        return l;
+    });
+
+    py::classh<B>(m, "B")
+        .def(py::init<>())
+        .def_readonly("val_b", &D::val_b)
+        .def("b", [](B *self) { return self; })
+        .def("get", [](const B &self) { return self.val_b; });
+
+    py::classh<C0, B>(m, "C0")
+        .def(py::init<>())
+        .def_readonly("val_c0", &D::val_c0)
+        .def("c0", [](C0 *self) { return self; })
+        .def("get", [](const C0 &self) { return self.val_b * 100 + self.val_c0; });
+
+    py::classh<C1, B>(m, "C1")
+        .def(py::init<>())
+        .def_readonly("val_c1", &D::val_c1)
+        .def("c1", [](C1 *self) { return self; })
+        .def("get", [](const C1 &self) { return self.val_b * 100 + self.val_c1; });
+
+    py::classh<D, C0, C1>(m, "D")
+        .def(py::init<>())
+        .def_readonly("val_d", &D::val_d)
+        .def("d", [](D *self) { return self; })
+        .def("get", [](const D &self) {
+            return self.val_b * 1000000 + self.val_c0 * 10000 + self.val_c1 * 100 + self.val_d;
+        });
+
+    m.def("disown_b", disown_b);
+}

tests/test_ccccccccccccccccccccc.py

@@ -0,0 +1,13 @@
+from __future__ import annotations
+
+import env  # noqa: F401
+from pybind11_tests import ccccccccccccccccccccc as m
+
+
+def test_disown_d():
+    d = m.D()
+    m.disown_b(d)
+
+
+def test_shptr_copy():
+    m.test_ShPtr_copy()