Introducing check_is_smart_holder_type_caster() function for runtime check,...

Introducing check_is_smart_holder_type_caster() function for runtime check, and reinterpreting record.default_holder as "uses_unique_ptr_holder". With this test_smart_ptr succeeds. (All 42 tests build, 35 tests succeed, 5 run but have some failures, 2 segfault.)

include/pybind11/cast.h

@@ -1201,11 +1201,18 @@ struct smart_holder_type_caster_class_hooks {
     }
 };
 
+template <typename T>
+inline bool check_is_smart_holder_type_caster();
+
 template <typename T>
 struct smart_holder_type_caster_load {
     using holder_type = pybindit::memory::smart_holder;
 
     bool load(handle src, bool convert) {
+        if (!check_is_smart_holder_type_caster<T>()) {
+            throw cast_error(
+                "Unable to load a smart-pointer type from a non-smart_holder instance.");
+        }
         load_impl = modified_type_caster_generic_load_impl(typeid(T));
         if (!load_impl.load(src, convert))
             return false;
@@ -2404,6 +2411,11 @@ struct is_smart_holder_type_caster<
     T,
     enable_if_t<type_caster<T>::is_smart_holder_type_caster::value, void>> : std::true_type {};
 
+template <typename T>
+inline bool check_is_smart_holder_type_caster() {
+    return detail::is_smart_holder_type_caster<T>::value;
+}
+
 // Detect whether returning a `type` from a cast on type's type_caster is going to result in a
 // reference or pointer to a local variable of the type_caster.  Basically, only
 // non-reference/pointer `type`s and reference/pointers from a type_caster_generic are safe;

include/pybind11/pybind11.h

@@ -1307,11 +1307,10 @@ public:
         record.holder_size = sizeof(holder_type);
         record.init_instance = init_instance;
         record.dealloc = dealloc;
-#ifndef PYBIND11_USE_SMART_HOLDER_AS_DEFAULT
+
+        // A better name would be uses_unique_ptr_holder.
         record.default_holder = detail::is_instantiation<std::unique_ptr, holder_type>::value;
-#else
-        record.default_holder = holder_is_smart_holder;
-#endif
+
         set_operator_new<type>(&record);
 
         /* Register base classes specified via template arguments to class_, if any */

tests/test_smart_ptr.py

@@ -302,10 +302,9 @@ def test_smart_ptr_from_default():
     instance = m.HeldByDefaultHolder()
     with pytest.raises(RuntimeError) as excinfo:
         m.HeldByDefaultHolder.load_shared_ptr(instance)
-    assert (
-        "Unable to load a custom holder type from a "
-        "default-holder instance" in str(excinfo.value)
-    )
+    assert str(excinfo.value) in (
+        "Unable to load a smart-pointer type from a non-smart_holder instance.",
+        "Unable to load a custom holder type from a default-holder instance")
 
 
 def test_shared_ptr_gc():