Introducing is_smart_holder_type_caster_base_tag, to keep smart_holder code more disconnected.

include/pybind11/cast.h

@@ -966,6 +966,12 @@ protected:
     static Constructor make_move_constructor(...) { return nullptr; }
 };
 
+// Tag to be used as base class, inspected by is_smart_holder_type_caster<T> test.
+struct is_smart_holder_type_caster_base_tag {};
+
+template <typename T>
+struct is_smart_holder_type_caster;
+
 //DETAIL/SMART_HOLDER_TYPE_CASTERS_H/BEGIN/////////////////////////////////////////////////////////
 
 //FWD begin
@@ -1192,7 +1198,7 @@ public:
 };
 // clang-format on
 
-struct smart_holder_type_caster_class_hooks {
+struct smart_holder_type_caster_class_hooks : is_smart_holder_type_caster_base_tag {
     static decltype(&modified_type_caster_generic_load_impl::local_load)
     get_local_load_function_ptr() {
         return &modified_type_caster_generic_load_impl::local_load;
@@ -1225,18 +1231,12 @@ 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.");
-        }
+        static_assert(is_smart_holder_type_caster<T>::value, "Internal consistency error.");
         load_impl = modified_type_caster_generic_load_impl(typeid(T));
         if (!load_impl.load(src, convert))
             return false;
@@ -1692,6 +1692,13 @@ template <typename type, typename SFINAE = void> class type_caster : public type
 
 template <typename type> using make_caster = type_caster<intrinsic_t<type>>;
 
+template <typename T>
+struct is_smart_holder_type_caster {
+    static constexpr bool value = std::is_base_of<
+        is_smart_holder_type_caster_base_tag,
+        make_caster<T>>::value;
+};
+
 // Shortcut for calling a caster's `cast_op_type` cast operator for casting a type_caster to a T
 template <typename T> typename make_caster<T>::template cast_op_type<T> cast_op(make_caster<T> &caster) {
     return caster.operator typename make_caster<T>::template cast_op_type<T>();
@@ -2459,21 +2466,6 @@ template <typename T> struct move_if_unreferenced<T, enable_if_t<all_of<
 >::value>> : std::true_type {};
 template <typename T> using move_never = none_of<move_always<T>, move_if_unreferenced<T>>;
 
-template <typename T, typename SFINAE = void>
-struct is_smart_holder_type_caster : std::false_type {};
-
-template <typename T>
-struct is_smart_holder_type_caster<
-    T,
-    typename std::enable_if<
-        std::is_base_of<smart_holder_type_caster_class_hooks, make_caster<T>>::value>::type>
-    : std::true_type {};
-
-template <typename T>
-inline bool check_is_smart_holder_type_caster() {
-    return 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;

tests/test_smart_ptr.cpp

@@ -292,6 +292,18 @@ PYBIND11_SMART_POINTER_HOLDER_TYPE_CASTERS(ElementBase, std::shared_ptr<ElementB
 PYBIND11_SMART_POINTER_HOLDER_TYPE_CASTERS(ElementA, std::shared_ptr<ElementA>)
 PYBIND11_SMART_POINTER_HOLDER_TYPE_CASTERS(ElementList, std::shared_ptr<ElementList>)
 
+#ifdef PYBIND11_USE_SMART_HOLDER_AS_DEFAULT
+// To prevent triggering a static_assert in the smart_holder code.
+// This is a very special case, because the associated test exercises a holder mismatch.
+namespace pybind11 {
+namespace detail {
+template <>
+class type_caster<std::shared_ptr<HeldByDefaultHolder>>
+    : public copyable_holder_caster<HeldByDefaultHolder, std::shared_ptr<HeldByDefaultHolder>> {};
+} // namespace detail
+} // namespace pybind11
+#endif
+
 TEST_SUBMODULE(smart_ptr, m) {
 
     // test_smart_ptr