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>

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()); });