Allow subclasses of py::args and py::kwargs (#5381)

* Allow subclasses of py::args and py::kwargs

The current implementation does not allow subclasses of args or kwargs.
This change allows subclasses to be used.

* Added test case

* style: pre-commit fixes

* Added missing semi-colons

* style: pre-commit fixes

* Added handle_type_name

* Moved classes outside of function

* Added namespaces

* style: pre-commit fixes

* Refactored tests

Added more tests and moved tests to more appropriate locations.

* style: pre-commit fixes

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

include/pybind11/cast.h

@@ -1570,9 +1570,9 @@ class argument_loader {
     using indices = make_index_sequence<sizeof...(Args)>;
 
     template <typename Arg>
-    using argument_is_args = std::is_same<intrinsic_t<Arg>, args>;
+    using argument_is_args = std::is_base_of<args, intrinsic_t<Arg>>;
     template <typename Arg>
-    using argument_is_kwargs = std::is_same<intrinsic_t<Arg>, kwargs>;
+    using argument_is_kwargs = std::is_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_kwargs_and_defaults.cpp

@@ -14,6 +14,26 @@
 
 #include <utility>
 
+// Classes needed for subclass test.
+class ArgsSubclass : public py::args {
+    using py::args::args;
+};
+class KWArgsSubclass : public py::kwargs {
+    using py::kwargs::kwargs;
+};
+namespace pybind11 {
+namespace detail {
+template <>
+struct handle_type_name<ArgsSubclass> {
+    static constexpr auto name = const_name("*Args");
+};
+template <>
+struct handle_type_name<KWArgsSubclass> {
+    static constexpr auto name = const_name("**KWArgs");
+};
+} // namespace detail
+} // namespace pybind11
+
 TEST_SUBMODULE(kwargs_and_defaults, m) {
     auto kw_func
         = [](int x, int y) { return "x=" + std::to_string(x) + ", y=" + std::to_string(y); };
@@ -322,4 +342,10 @@ TEST_SUBMODULE(kwargs_and_defaults, m) {
             py::pos_only{},
             py::arg("i"),
             py::arg("j"));
+
+    // Test support for args and kwargs subclasses
+    m.def("args_kwargs_subclass_function",
+          [](const ArgsSubclass &args, const KWArgsSubclass &kwargs) {
+              return py::make_tuple(args, kwargs);
+          });
 }

tests/test_kwargs_and_defaults.py

@@ -17,6 +17,10 @@ def test_function_signatures(doc):
     assert (
         doc(m.args_kwargs_function) == "args_kwargs_function(*args, **kwargs) -> tuple"
     )
+    assert (
+        doc(m.args_kwargs_subclass_function)
+        == "args_kwargs_subclass_function(*Args, **KWArgs) -> tuple"
+    )
     assert (
         doc(m.KWClass.foo0)
         == "foo0(self: m.kwargs_and_defaults.KWClass, arg0: int, arg1: float) -> None"
@@ -98,6 +102,7 @@ def test_arg_and_kwargs():
     args = "a1", "a2"
     kwargs = {"arg3": "a3", "arg4": 4}
     assert m.args_kwargs_function(*args, **kwargs) == (args, kwargs)
+    assert m.args_kwargs_subclass_function(*args, **kwargs) == (args, kwargs)
 
 
 def test_mixed_args_and_kwargs(msg):
@@ -413,6 +418,12 @@ def test_args_refcount():
     )
     assert refcount(myval) == expected
 
+    assert m.args_kwargs_subclass_function(7, 8, myval, a=1, b=myval) == (
+        (7, 8, myval),
+        {"a": 1, "b": myval},
+    )
+    assert refcount(myval) == expected
+
     exp3 = refcount(myval, myval, myval)
     assert m.args_refcount(myval, myval, myval) == (exp3, exp3, exp3)
     assert refcount(myval) == expected