Adding `test_pass_released_shared_ptr_as_unique_ptr`, exercising new guard in smart_holder_type_casters.h.

include/pybind11/detail/smart_holder_type_casters.h

@@ -438,6 +438,7 @@ to_cout("LOOOK loaded_as_shared_ptr return hld vptr " + std::to_string(__LINE__)
         if (!have_holder())
             return nullptr;
         throw_if_uninitialized_or_disowned_holder();
+        throw_if_instance_is_currently_owned_by_shared_ptr();
         holder().ensure_is_not_disowned(context);
         holder().template ensure_compatible_rtti_uqp_del<T, D>(context);
         holder().ensure_use_count_1(context);
@@ -501,6 +502,13 @@ private:
         }
     }
 
+    // have_holder() must be true or this function will fail.
+    void throw_if_instance_is_currently_owned_by_shared_ptr() const {
+        if (holder().vptr_is_released) {
+            throw value_error("Python instance is currently owned by a std::shared_ptr.");
+        }
+    }
+
     T *convert_type(void *void_ptr) const {
         if (void_ptr != nullptr && load_impl.loaded_v_h_cpptype != nullptr
             && !load_impl.reinterpret_cast_deemed_ok && load_impl.implicit_cast != nullptr) {

tests/test_class_sh_trampoline_shared_from_this.cpp

@@ -70,7 +70,7 @@ struct SftSharedPtrStash {
     }
 };
 
-struct SftTrampoline : Sft {
+struct SftTrampoline : Sft, py::trampoline_self_life_support {
     using Sft::Sft;
 };
 
@@ -78,6 +78,8 @@ void pass_shared_ptr(const std::shared_ptr<Sft> &obj) {
     obj->shared_from_this()->history += "_PassSharedPtr";
 }
 
+void pass_unique_ptr(const std::unique_ptr<Sft> &) {}
+
 } // namespace
 
 PYBIND11_SMART_HOLDER_TYPE_CASTERS(Sft)
@@ -97,4 +99,5 @@ TEST_SUBMODULE(class_sh_trampoline_shared_from_this, m) {
         .def("use_count", &SftSharedPtrStash::use_count);
 
     m.def("pass_shared_ptr", pass_shared_ptr);
+    m.def("pass_unique_ptr", pass_unique_ptr);
 }

tests/test_class_sh_trampoline_shared_from_this.py

@@ -1,4 +1,5 @@
 # -*- coding: utf-8 -*-
+import pytest
 
 import env  # noqa: F401
 
@@ -80,3 +81,14 @@ def test_pass_shared_ptr_while_stashed_with_shared_from_this():
     gc.collect()
     if not env.PYPY:
         assert obj_wr() is None
+
+
+def test_pass_released_shared_ptr_as_unique_ptr():
+    obj = PySft("PySft")
+    stash1 = m.SftSharedPtrStash(1)
+    stash1.Add(obj)  # Releases shared_ptr to C++.
+    with pytest.raises(ValueError) as exc_info:
+        m.pass_unique_ptr(obj)
+    assert str(exc_info.value) == (
+        "Python instance is currently owned by a std::shared_ptr."
+    )