Adding unit test.

include/pybind11/detail/init.h

@@ -298,7 +298,7 @@ void setstate(value_and_holder &v_h, std::pair<T, O> &&result, bool need_alias)
         // Skipping setattr below, to not force use of py::dynamic_attr() for Class unnecessarily.
         return;
     }
-    setattr((PyObject *) v_h.inst, "__dict__", result.second);
+    setattr((PyObject *) v_h.inst, "__dict__", d);
 }
 
 /// Implementation for py::pickle(GetState, SetState)

tests/CMakeLists.txt

@@ -123,6 +123,7 @@ set(PYBIND11_TEST_FILES
     test_opaque_types.cpp
     test_operator_overloading.cpp
     test_pickling.cpp
+    test_pickling_trampoline.cpp
     test_pytypes.cpp
     test_sequences_and_iterators.cpp
     test_smart_ptr.cpp

tests/test_pickling_trampoline.cpp

@@ -0,0 +1,45 @@
+// Copyright (c) 2021 The Pybind Development Team.
+// All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+#include "pybind11_tests.h"
+
+#include <memory>
+#include <utility>
+
+namespace {
+
+struct SimpleBase {
+    int num               = 0;
+    virtual ~SimpleBase() = default;
+};
+
+struct SimpleBaseTrampoline : SimpleBase {};
+
+struct SimpleCppDerived : SimpleBase {};
+
+} // namespace
+
+TEST_SUBMODULE(pickling_trampoline, m) {
+    py::class_<SimpleBase, SimpleBaseTrampoline>(m, "SimpleBase")
+        .def(py::init<>())
+        .def_readwrite("num", &SimpleBase::num)
+        .def(py::pickle(
+            [](py::object self) {
+                py::dict d;
+                if (py::hasattr(self, "__dict__"))
+                    d = self.attr("__dict__");
+                return py::make_tuple(self.attr("num"), d);
+            },
+            [](py::tuple t) {
+                if (t.size() != 2)
+                    throw std::runtime_error("Invalid state!");
+                auto cpp_state = std::unique_ptr<SimpleBase>(new SimpleBaseTrampoline);
+                cpp_state->num = t[0].cast<int>();
+                auto py_state  = t[1].cast<py::dict>();
+                return std::make_pair(std::move(cpp_state), py_state);
+            }));
+
+    m.def("make_SimpleCppDerivedAsBase",
+          []() { return std::unique_ptr<SimpleBase>(new SimpleCppDerived); });
+}

tests/test_pickling_trampoline.py

@@ -0,0 +1,34 @@
+# -*- coding: utf-8 -*-
+import pytest
+
+from pybind11_tests import pickling_trampoline as m
+
+try:
+    import cPickle as pickle  # Use cPickle on Python 2.7
+except ImportError:
+    import pickle
+
+
+class SimplePyDerived(m.SimpleBase):
+    pass
+
+
+def test_roundtrip_simple_py_derived():
+    p = SimplePyDerived()
+    p.num = 202
+    p.stored_in_dict = 303
+    data = pickle.dumps(p, pickle.HIGHEST_PROTOCOL)
+    p2 = pickle.loads(data)
+    assert p2.num == 202
+    assert p2.stored_in_dict == 303
+
+
+def test_roundtrip_simple_cpp_derived():
+    p = m.make_SimpleCppDerivedAsBase()
+    p.num = 404
+    with pytest.raises(AttributeError):
+        # To ensure that future changes do not accidentally invalidate this unit test.
+        p.__dict__
+    data = pickle.dumps(p, pickle.HIGHEST_PROTOCOL)
+    p2 = pickle.loads(data)
+    assert p2.num == 404