HELP WANTED: type_caster_incomplete_type sketch

tests/CMakeLists.txt

@@ -149,6 +149,7 @@ set(PYBIND11_TEST_FILES
     test_stl_binders
     test_tagbased_polymorphic
     test_thread
+    test_type_caster_incomplete_type
     test_type_caster_pyobject_ptr
     test_type_caster_std_function_specializations
     test_union

tests/test_type_caster_incomplete_type.cpp

@@ -0,0 +1,47 @@
+#include "pybind11_tests.h"
+
+namespace test_type_caster_incomplete_type {
+
+struct ForwardDeclaredType {};
+
+} // namespace test_type_caster_incomplete_type
+
+using ForwardDeclaredType = test_type_caster_incomplete_type::ForwardDeclaredType;
+
+// TODO: Move to pybind11/type_caster_incomplete_type.h, wrap in a macro.
+namespace pybind11 {
+namespace detail {
+
+template <>
+class type_caster<ForwardDeclaredType> {
+public:
+    static constexpr auto name = const_name("object");
+
+    static handle
+    cast(ForwardDeclaredType * /*src*/, return_value_policy /*policy*/, handle /*parent*/) {
+        return py::none().release(); // TODO: Build and return capsule with src pointer;
+    }
+
+    bool load(handle /*src*/, bool /*convert*/) {
+        // TODO: Assign pointer_capsule = src after inspecting src.
+        return true;
+    }
+
+    template <typename T>
+    using cast_op_type = ForwardDeclaredType *;
+
+    explicit operator ForwardDeclaredType *() {
+        return nullptr; // TODO: Retrieve C++ pointer from pointer_capsule.
+    }
+
+private:
+    capsule pointer_capsule;
+};
+
+} // namespace detail
+} // namespace pybind11
+
+TEST_SUBMODULE(type_caster_incomplete_type, m) {
+    m.def("rtrn_fwd_decl_type_ptr", []() { return reinterpret_cast<ForwardDeclaredType *>(0); });
+    m.def("pass_fwd_decl_type_ptr", [](ForwardDeclaredType *) {});
+}

tests/test_type_caster_incomplete_type.py

@@ -0,0 +1,11 @@
+from __future__ import annotations
+
+from pybind11_tests import type_caster_incomplete_type as m
+
+
+def test_rtrn_fwd_decl_type_ptr():
+    assert m.rtrn_fwd_decl_type_ptr() is None
+
+
+def test_pass_fwd_decl_type_ptr():
+    assert m.pass_fwd_decl_type_ptr(None) is None