only check ref_count for temporary values

include/pybind11/cast.h

@@ -1219,17 +1219,19 @@ move(T &&value) {
 }
 
 template <typename T>
-detail::enable_if_t<!detail::move_never<T>::value, T> move(object &&obj) {
-    if (obj.ref_count() > 1) {
+detail::enable_if_t<!detail::move_never<T>::value, T> move(object &&obj) {\
+    if constexpr (detail::cast_is_temporary_value_reference<T>::value) {
+        if (obj.ref_count() > 1) {
 #if !defined(PYBIND11_DETAILED_ERROR_MESSAGES)
-        throw cast_error(
+            throw cast_error(
             "Unable to cast Python instance to C++ rvalue: instance has multiple references"
-            " (#define PYBIND11_DETAILED_ERROR_MESSAGES or compile in debug mode for details)");
+                " (#define PYBIND11_DETAILED_ERROR_MESSAGES or compile in debug mode for details)");
 #else
-        throw cast_error("Unable to move from Python " + (std::string) str(type::handle_of(obj))
-                         + " instance to C++ " + type_id<T>()
+            throw cast_error("Unable to move from Python " + (std::string) str(type::handle_of(obj))
+                             + " instance to C++ " + type_id<T>()
                          + " instance: instance has multiple references");
 #endif
+        }
     }
 
     // Move into a temporary and return that, because the reference may be a local value of `conv`

tests/test_smart_ptr.cpp

@@ -10,6 +10,7 @@
 
 #include "object.h"
 #include "pybind11_tests.h"
+#include "pybind11/functional.h"
 
 namespace {
 
@@ -531,6 +532,16 @@ TEST_SUBMODULE(smart_ptr, m) {
         return out;
     });
 
+    using Callback = std::function<std::unique_ptr<UniquePtrHeld>()>;
+    m.def("unique_ptr_callback",
+        [](const Callback& callback) {
+            auto obj = callback();
+            if (obj->value() != 10) {
+                throw std::runtime_error("obj.value() must be 10");
+            }
+            return callback();
+        });
+
     // Ensure class is non-empty, so it's easier to detect double-free
     // corruption. (If empty, this may be harder to see easily.)
     struct SharedPtrHeld {

tests/test_smart_ptr.py

@@ -378,3 +378,16 @@ def check_reset(obj_new):
 
     check_reset(obj_new=m.UniquePtrHeld(10))
     check_reset(obj_new=None)
+
+
+def test_unique_ptr_self_reference():
+    obj = None
+
+    def make_unique_ptr_held():
+        nonlocal obj
+        # This simulates a self-reference for a derived type.
+        obj = m.UniquePtrHeld(10)
+        return obj
+
+    out = m.unique_ptr_callback(make_unique_ptr_held)
+    assert obj is out

tests/test_virtual_functions.py

@@ -216,7 +216,10 @@ def get_movable(self, a, b):
     ncv2 = NCVirtExt2()
     assert ncv2.print_movable(7, 7) == "14"
     # Don't check the exception message here because it differs under debug/non-debug mode
-    with pytest.raises(RuntimeError):
+
+    # N.B. Drake fork allows this behavior.
+    if True:
+    # with pytest.raises(RuntimeError):
         ncv2.print_nc(9, 9)
 
     nc_stats = ConstructorStats.get(m.NonCopyable)