Porting additional demonstration of pybind11 Undefined Behavior in ha…

…ndling of shared_ptr holder.

test_make_drvd_up_cast_pass_drvd fails with pybind11 (segfault) but passes with Boost.Python.

The root cause is the explicit cast in the pybind11 code here:
https://github.com/pybind/pybind11/blob/30eb39ed79d1e2eeff15219ac00773034300a5e6/include/pybind11/pybind11.h#L1505
    init_holder(inst, v_h, (const holder_type *) holder_ptr, v_h.value_ptr<type>());

smart_ptr_private_first_base_ext.cpp

@@ -30,7 +30,12 @@ inline std::shared_ptr<drvd> make_shared_drvd() {
   return std::shared_ptr<drvd>(new drvd);
 }
 
+inline std::shared_ptr<base> make_shared_drvd_up_cast() {
+  return std::shared_ptr<base>(new drvd);
+}
+
 inline int pass_shared_base(std::shared_ptr<base> b) { return b->id(); }
+inline int pass_shared_drvd(std::shared_ptr<drvd> d) { return d->id(); }
 
 }  // namespace
 
@@ -42,5 +47,7 @@ BOOST_PYTHON_MODULE(rwgk_tbx_smart_ptr_private_first_base_ext)
   py::class_<drvd, py::bases<base>, std::shared_ptr<drvd>>("drvd");
 
   py::def("make_shared_drvd", make_shared_drvd);
+  py::def("make_shared_drvd_up_cast", make_shared_drvd_up_cast);
   py::def("pass_shared_base", pass_shared_base);
+  py::def("pass_shared_drvd", pass_shared_drvd);
 }

tst_smart_ptr_private_first_base.py

@@ -3,12 +3,27 @@
 import rwgk_tbx.smart_ptr_private_first_base as m
 import sys
 
+
+def test_make_drvd_pass_base():
+    d = m.make_shared_drvd()
+    i = m.pass_shared_base(d)
+    assert i == 200
+
+
+def test_make_drvd_up_cast_pass_drvd():
+    b = m.make_shared_drvd_up_cast()
+    # the base return is up-cast immediately.
+    assert b.__class__.__name__ == "drvd"
+    i = m.pass_shared_drvd(b)
+    assert i == 200
+
+
 def run(args):
   assert not args
-  d = m.make_shared_drvd()
-  i = m.pass_shared_base(d)
-  assert i == 200
+  test_make_drvd_pass_base()
+  test_make_drvd_up_cast_pass_drvd()
   print("Done.")
 
+
 if (__name__ == "__main__"):
   run(sys.argv[1:])