Content of PR pybind#4374 applied on top of smart_holder branch.

Author: rwgk

tests/CMakeLists.txt

@@ -158,6 +158,7 @@ set(PYBIND11_TEST_FILES
     test_kwargs_and_defaults
     test_local_bindings
     test_methods_and_attributes
+    test_mi_debug
     test_modules
     test_multiple_inheritance
     test_numpy_array

tests/test_mi_debug.cpp

@@ -0,0 +1,53 @@
+#include <pybind11/pybind11.h>
+
+#include "pybind11_tests.h"
+
+#include <iostream>
+#include <memory>
+#include <vector>
+
+// The first base class.
+struct Base0 {
+    virtual ~Base0() = default;
+};
+
+using Base0Ptr = std::shared_ptr<Base0>;
+
+// The second base class.
+struct Base1 {
+    virtual ~Base1() = default;
+    std::vector<int> vec = {1, 2, 3, 4, 5};
+};
+
+using Base1Ptr = std::shared_ptr<Base1>;
+
+// The derived class.
+struct Derived : Base1, Base0 {
+    ~Derived() override = default;
+};
+
+using DerivedPtr = std::shared_ptr<Derived>;
+
+TEST_SUBMODULE(mi_debug, m) {
+    // Expose the bases.
+    pybind11::class_<Base0, Base0Ptr> bs0(m, "Base0");
+    pybind11::class_<Base1, Base1Ptr> bs1(m, "Base1");
+    // Expose the derived class.
+    pybind11::class_<Derived, DerivedPtr, Base0, Base1>(m, "Derived").def(pybind11::init<>());
+
+    // A helper that returns a pointer to base.
+    m.def("make_object", []() -> Base0Ptr {
+        auto ret_der = std::make_shared<Derived>();
+        std::cout << "ret der ptr: " << ret_der.get() << std::endl;
+        auto ret = Base0Ptr(ret_der);
+        std::cout << "ret base ptr: " << ret.get() << std::endl;
+        return ret;
+    });
+
+    // A helper that accepts in input a pointer to derived.
+    m.def("get_object_vec_size", [](const DerivedPtr &object) {
+        std::cout << "der ptr: " << object.get() << std::endl;
+        std::cout << object->vec.size() << std::endl;
+        return object->vec.size();
+    });
+}

tests/test_mi_debug.py

@@ -0,0 +1,8 @@
+import pytest
+
+m = pytest.importorskip("pybind11_tests.mi_debug")
+
+
+def test_vec():
+    o = m.make_object()
+    assert 5 == m.get_object_vec_size(o)