feat(types): add support for Typing.Callable Special Case (#5202)

* Add special case

* linty

Author: InvincibleRMC

include/pybind11/typing.h

@@ -177,6 +177,14 @@ struct handle_type_name<typing::Callable<Return(Args...)>> {
           + const_name("], ") + make_caster<retval_type>::name + const_name("]");
 };
 
+template <typename Return>
+struct handle_type_name<typing::Callable<Return(ellipsis)>> {
+    // PEP 484 specifies this syntax for defining only return types of callables
+    using retval_type = conditional_t<std::is_same<Return, void>::value, void_type, Return>;
+    static constexpr auto name
+        = const_name("Callable[..., ") + make_caster<retval_type>::name + const_name("]");
+};
+
 template <typename T>
 struct handle_type_name<typing::Type<T>> {
     static constexpr auto name = const_name("type[") + make_caster<T>::name + const_name("]");

tests/test_pytypes.cpp

@@ -865,6 +865,7 @@ TEST_SUBMODULE(pytypes, m) {
     m.def("annotate_fn",
           [](const py::typing::Callable<int(py::typing::List<py::str>, py::str)> &) {});
 
+    m.def("annotate_fn_only_return", [](const py::typing::Callable<int(py::ellipsis)> &) {});
     m.def("annotate_type", [](const py::typing::Type<int> &t) -> py::type { return t; });
 
     m.def("annotate_union",

tests/test_pytypes.py

@@ -959,6 +959,13 @@ def test_fn_annotations(doc):
     )
 
 
+def test_fn_return_only(doc):
+    assert (
+        doc(m.annotate_fn_only_return)
+        == "annotate_fn_only_return(arg0: Callable[..., int]) -> None"
+    )
+
+
 def test_type_annotation(doc):
     assert doc(m.annotate_type) == "annotate_type(arg0: type[int]) -> type"