Fix refcount bug involving trampoline functions with `PyObject *` return type. (#5156)

* Transfer diff from pybind11k fork as-is. New tests are still missing.

* Add `PYBIND11_WARNING_DISABLE_MSVC(4127)` into `PYBIND11_OVERRIDE_IMPL` macro.

* Add test_trampoline_with_pyobject_ptr_return()

* Resolve clang-tidy error: use auto when initializing with a template cast to avoid duplicating the type name [modernize-use-auto,-warnings-as-errors]

* Disabled checking refcount when building with PyPy.

* Clang 3.6, 3.7, 3.9 compatibility.

```
/__w/pybind11/pybind11/tests/test_type_caster_pyobject_ptr.cpp:23:13: error: definition of implicit copy constructor for 'WithPyObjectPtrReturn' is deprecated because it has a user-declared destructor [-Werror,-Wdeprecated]
    virtual ~WithPyObjectPtrReturn() = default;
            ^
```

* Minor clean-up of production code changes.

* Add missing `override` (to resolve clang-tidy error).

* Move PYBIND11_WARNING_POP for a better clang-format outcome.

include/pybind11/cast.h

@@ -1339,13 +1339,24 @@ enable_if_t<!cast_is_temporary_value_reference<T>::value, T> cast_ref(object &&,
 // static_assert, even though if it's in dead code, so we provide a "trampoline" to pybind11::cast
 // that only does anything in cases where pybind11::cast is valid.
 template <typename T>
-enable_if_t<cast_is_temporary_value_reference<T>::value, T> cast_safe(object &&) {
+enable_if_t<cast_is_temporary_value_reference<T>::value
+                && !detail::is_same_ignoring_cvref<T, PyObject *>::value,
+            T>
+cast_safe(object &&) {
     pybind11_fail("Internal error: cast_safe fallback invoked");
 }
 template <typename T>
 enable_if_t<std::is_void<T>::value, void> cast_safe(object &&) {}
 template <typename T>
-enable_if_t<detail::none_of<cast_is_temporary_value_reference<T>, std::is_void<T>>::value, T>
+enable_if_t<detail::is_same_ignoring_cvref<T, PyObject *>::value, PyObject *>
+cast_safe(object &&o) {
+    return o.release().ptr();
+}
+template <typename T>
+enable_if_t<detail::none_of<cast_is_temporary_value_reference<T>,
+                            detail::is_same_ignoring_cvref<T, PyObject *>,
+                            std::is_void<T>>::value,
+            T>
 cast_safe(object &&o) {
     return pybind11::cast<T>(std::move(o));
 }

include/pybind11/pybind11.h

@@ -2868,10 +2868,14 @@ function get_override(const T *this_ptr, const char *name) {
             = pybind11::get_override(static_cast<const cname *>(this), name);                     \
         if (override) {                                                                           \
             auto o = override(__VA_ARGS__);                                                       \
-            if (pybind11::detail::cast_is_temporary_value_reference<ret_type>::value) {           \
+            PYBIND11_WARNING_PUSH                                                                 \
+            PYBIND11_WARNING_DISABLE_MSVC(4127)                                                   \
+            if (pybind11::detail::cast_is_temporary_value_reference<ret_type>::value              \
+                && !pybind11::detail::is_same_ignoring_cvref<ret_type, PyObject *>::value) {      \
                 static pybind11::detail::override_caster_t<ret_type> caster;                      \
                 return pybind11::detail::cast_ref<ret_type>(std::move(o), caster);                \
             }                                                                                     \
+            PYBIND11_WARNING_POP                                                                  \
             return pybind11::detail::cast_safe<ret_type>(std::move(o));                           \
         }                                                                                         \
     } while (false)

tests/test_type_caster_pyobject_ptr.cpp

@@ -5,9 +5,10 @@
 #include "pybind11_tests.h"
 
 #include <cstddef>
+#include <string>
 #include <vector>
 
-namespace {
+namespace test_type_caster_pyobject_ptr {
 
 std::vector<PyObject *> make_vector_pyobject_ptr(const py::object &ValueHolder) {
     std::vector<PyObject *> vec_obj;
@@ -18,9 +19,39 @@ std::vector<PyObject *> make_vector_pyobject_ptr(const py::object &ValueHolder)
     return vec_obj;
 }
 
-} // namespace
+struct WithPyObjectPtrReturn {
+#if defined(__clang_major__) && __clang_major__ < 4
+    WithPyObjectPtrReturn() = default;
+    WithPyObjectPtrReturn(const WithPyObjectPtrReturn &) = default;
+#endif
+    virtual ~WithPyObjectPtrReturn() = default;
+    virtual PyObject *return_pyobject_ptr() const = 0;
+};
+
+struct WithPyObjectPtrReturnTrampoline : WithPyObjectPtrReturn {
+    PyObject *return_pyobject_ptr() const override {
+        PYBIND11_OVERRIDE_PURE(PyObject *, WithPyObjectPtrReturn, return_pyobject_ptr,
+                               /* no arguments */);
+    }
+};
+
+std::string call_return_pyobject_ptr(const WithPyObjectPtrReturn *base_class_ptr) {
+    PyObject *returned_obj = base_class_ptr->return_pyobject_ptr();
+#if !defined(PYPY_VERSION) // It is not worth the trouble doing something special for PyPy.
+    if (Py_REFCNT(returned_obj) != 1) {
+        py::pybind11_fail(__FILE__ ":" PYBIND11_TOSTRING(__LINE__));
+    }
+#endif
+    auto ret_val = py::repr(returned_obj).cast<std::string>();
+    Py_DECREF(returned_obj);
+    return ret_val;
+}
+
+} // namespace test_type_caster_pyobject_ptr
 
 TEST_SUBMODULE(type_caster_pyobject_ptr, m) {
+    using namespace test_type_caster_pyobject_ptr;
+
     m.def("cast_from_pyobject_ptr", []() {
         PyObject *ptr = PyLong_FromLongLong(6758L);
         return py::cast(ptr, py::return_value_policy::take_ownership);
@@ -127,4 +158,10 @@ TEST_SUBMODULE(type_caster_pyobject_ptr, m) {
         (void) py::cast(*ptr);
     }
 #endif
+
+    py::class_<WithPyObjectPtrReturn, WithPyObjectPtrReturnTrampoline>(m, "WithPyObjectPtrReturn")
+        .def(py::init<>())
+        .def("return_pyobject_ptr", &WithPyObjectPtrReturn::return_pyobject_ptr);
+
+    m.def("call_return_pyobject_ptr", call_return_pyobject_ptr);
 }

tests/test_type_caster_pyobject_ptr.py

@@ -102,3 +102,19 @@ def test_return_list_pyobject_ptr_reference():
 def test_type_caster_name_via_incompatible_function_arguments_type_error():
     with pytest.raises(TypeError, match=r"1\. \(arg0: object, arg1: int\) -> None"):
         m.pass_pyobject_ptr_and_int(ValueHolder(101), ValueHolder(202))
+
+
+def test_trampoline_with_pyobject_ptr_return():
+    class Drvd(m.WithPyObjectPtrReturn):
+        def return_pyobject_ptr(self):
+            return ["11", "22", "33"]
+
+    # Basic health check: First make sure this works as expected.
+    d = Drvd()
+    assert d.return_pyobject_ptr() == ["11", "22", "33"]
+
+    while True:
+        # This failed before PR #5156: AddressSanitizer: heap-use-after-free ... in Py_DECREF
+        d_repr = m.call_return_pyobject_ptr(d)
+        assert d_repr == repr(["11", "22", "33"])
+        break  # Comment out for manual leak checking.