Minimal reproducer for breakage introduced by PR #3402.

tests/test_kwargs_and_defaults.cpp

@@ -167,4 +167,39 @@ TEST_SUBMODULE(kwargs_and_defaults, m) {
         "class_default_argument",
         [](py::object a) { return py::repr(std::move(a)); },
         "a"_a = py::module_::import("decimal").attr("Decimal"));
+
+    m.def(
+        "func_kw_only_0", [](int) {}, py::kw_only(), py::arg("i") = 0);
+    m.def(
+        "func_kw_only_1", [](int, int) {}, py::arg("a") = 0, py::kw_only(), py::arg("i") = 0);
+    m.def(
+        "func_kw_only_2",
+        [](int, int, int) {},
+        py::arg("a") = 0,
+        py::arg("b") = 0,
+        py::kw_only(),
+        py::arg("i") = 0);
+
+    // https://github.com/pybind/pybind11/pull/3402#issuecomment-963341987
+    // Originally reduced from tensorstore.Dim
+    struct init_kw_only_0 {};
+    py::class_<init_kw_only_0>(m, "init_kw_only_0")
+        .def(py::init([](int) { return init_kw_only_0(); }),
+             py::kw_only(), // test passes with this line commented out.
+             py::arg("i") = 0);
+
+    struct init_kw_only_1 {};
+    py::class_<init_kw_only_1>(m, "init_kw_only_1")
+        .def(py::init([](int, int) { return init_kw_only_1(); }),
+             py::arg("a") = 0,
+             py::kw_only(), // test passes with this line commented out.
+             py::arg("i") = 0);
+
+    struct init_kw_only_2 {};
+    py::class_<init_kw_only_2>(m, "init_kw_only_2")
+        .def(py::init([](int, int, int) { return init_kw_only_2(); }),
+             py::arg("a") = 0,
+             py::arg("b") = 0,
+             py::kw_only(), // test passes with this line commented out.
+             py::arg("i") = 0);
 }

tests/test_kwargs_and_defaults.py

@@ -370,3 +370,27 @@ def test_args_refcount():
     assert m.mixed_args_refcount(myval, myval, myval) == (exp3 + 3, exp3 + 3, exp3 + 3)
 
     assert m.class_default_argument() == "<class 'decimal.Decimal'>"
+
+
+@pytest.mark.parametrize(
+    "func",
+    [
+        m.func_kw_only_0,
+        m.func_kw_only_1,
+        m.func_kw_only_2,
+    ],
+)
+def test_func_kw_only(func):
+    func(i=1)
+
+
+@pytest.mark.parametrize(
+    "init",
+    [
+        m.init_kw_only_0,
+        m.init_kw_only_1,
+        m.init_kw_only_2,
+    ],
+)
+def test_init_kw_only(init):
+    init(i=1)