Demonstration of Undefined Behavior in handling of shared_ptr holder.

Based on https://godbolt.org/z/4fdjaW by jorgbrown@ (thanks Jorg!).

tests/test_smart_ptr_private_first_base.cpp

+// Demonstration of Undefined Behavior in handling of shared_ptr holder,
+// specifically:
+// https://github.com/pybind/pybind11/blob/30eb39ed79d1e2eeff15219ac00773034300a5e6/include/pybind11/cast.h#L235
+//     `return reinterpret_cast<H &>(vh[1]);`
+// indirectly casts a `shared_ptr<drvd>` reference to a `shared_ptr<base>`.
+// `test_smart_ptr_private_first_base.py` fails with an AssertionError and
+// a subsequent Segmentation Fault (Linux, clang++ -std=c++17).
+
+#include <memory>
+
+#include "pybind11_tests.h"
+
+namespace pybind11_tests {
+namespace smart_ptr_private_first_base {
+
+struct base {
+  base() : base_id(100) {}
+  virtual ~base() = default;
+  virtual int id() const { return base_id; }
+  int base_id;
+};
+
+struct private_first_base {  // Any class with a virtual function will do.
+  virtual void some_other_virtual_function() const {}
+  virtual ~private_first_base() = default;
+};
+
+struct drvd : private private_first_base, public base {
+  int id() const override { return 2 * base_id; }
+};
+
+inline std::shared_ptr<drvd> make_shared_drvd() {
+  return std::shared_ptr<drvd>(new drvd);
+}
+
+inline int pass_shared_base(std::shared_ptr<base> b) { return b->id(); }
+
+TEST_SUBMODULE(smart_ptr_private_first_base, m) {
+  py::class_<base, std::shared_ptr<base>>(m, "base");
+  py::class_<drvd, base, std::shared_ptr<drvd>>(m, "drvd");
+
+  m.def("make_shared_drvd", make_shared_drvd);
+  m.def("pass_shared_base", pass_shared_base);
+}
+
+}  // namespace smart_ptr_private_first_base
+}  // namespace pybind11_tests

tests/test_smart_ptr_private_first_base.py

+# -*- coding: utf-8 -*-
+import pytest
+
+from pybind11_tests import smart_ptr_private_first_base as m
+
+def test_make_pass():
+    d = m.make_shared_drvd()
+    i = m.pass_shared_base(d)
+    assert i == 200