Address regression introduced in #5381 (#5396)

* Incomplete attempt to address regression introduced in #5381

* style: pre-commit fixes

* Revert "style: pre-commit fixes"

This reverts commit 9d107d2f75.

* Revert "Incomplete attempt to address regression introduced in #5381"

This reverts commit 8cf1cdbc96.

* Simpler fix for the regression introduced in #5381

* style: pre-commit fixes

* Added if constexpr workaround

This can probably be done better but at least this is a start.

* style: pre-commit fixes

* Replace if constexpr with template struct

if constexpr was not added until C++ 17.
I think this should do the same thing as before.

* style: pre-commit fixes

* Made comment clearer

* Added test cases

* style: pre-commit fixes

* Fixed is_same_or_base_of reference

* style: pre-commit fixes

* Added static assert messages

* style: pre-commit fixes

* Replaced typedef with using

* style: pre-commit fixes

* Back out `ForwardClassPtr` (to be discussed separately). Tested locally with clang-tidy.

* Shuffle new `static_assert()` and leave error messages blank (they are more distracting than helpful here).

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: gentlegiantJGC <gentlegiantJGC@users.noreply.github.com>
Co-authored-by: Ralf W. Grosse-Kunstleve <rgrossekunst@nvidia.com>

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.
+// is_base_of raises a compiler error if Derived is incomplete.
+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...>();
 

tests/test_class.cpp

@@ -52,8 +52,24 @@ void bind_empty0(py::module_ &m) {
 }
 
 } // namespace pr4220_tripped_over_this
+
+namespace pr5396_forward_declared_class {
+class ForwardClass;
+class Args : public py::args {};
+} // namespace pr5396_forward_declared_class
+
 } // namespace test_class
 
+static_assert(py::detail::is_same_or_base_of<py::args, py::args>::value, "");
+static_assert(
+    py::detail::is_same_or_base_of<py::args,
+                                   test_class::pr5396_forward_declared_class::Args>::value,
+    "");
+static_assert(!py::detail::is_same_or_base_of<
+                  py::args,
+                  test_class::pr5396_forward_declared_class::ForwardClass>::value,
+              "");
+
 TEST_SUBMODULE(class_, m) {
     m.def("obj_class_name", [](py::handle obj) { return py::detail::obj_class_name(obj.ptr()); });