Address regression introduced in #5381

include/pybind11/cast.h

@@ -1564,15 +1564,24 @@ struct function_call {
     handle init_self;
 };
 
+// See PR #5396 for the discussion that led to this
+template <typename Base, typename Derived, typename = void>
+struct is_same_or_base_of : std::is_same<Base, Derived> {};
+
+// Only evaluate is_base_of if Derived is complete.
+// It will raise a compiler error if Derived is not complete.
+template <typename Base, typename Derived>
+struct is_same_or_base_of<Base, Derived, decltype(void(sizeof(Derived)))>
+    : any_of<std::is_same<Base, Derived>, std::is_base_of<Base, Derived>> {};
+
 /// Helper class which loads arguments for C++ functions called from Python
 template <typename... Args>
 class argument_loader {
     using indices = make_index_sequence<sizeof...(Args)>;
-
     template <typename Arg>
-    using argument_is_args = std::is_base_of<args, intrinsic_t<Arg>>;
+    using argument_is_args = is_same_or_base_of<args, intrinsic_t<Arg>>;
     template <typename Arg>
-    using argument_is_kwargs = std::is_base_of<kwargs, intrinsic_t<Arg>>;
+    using argument_is_kwargs = is_same_or_base_of<kwargs, intrinsic_t<Arg>>;
     // Get kwargs argument position, or -1 if not present:
     static constexpr auto kwargs_pos = constexpr_last<argument_is_kwargs, Args...>();