Equivalent of PR pybind#4597, but based on pybind11k main @ ab472bbce81de4e0ae41d861a5c46903968f7b27

The pybind11k `return_value_policy_pack` and `function_record` changes were backed out.

The net diffs to PR pybind#4597 are merely:

```diff
+#define PYBIND11_HAS_TYPE_CASTER_STD_FUNCTION_SPECIALIZATIONS
```

```diff
-    explicit func_wrapper_base(func_handle &&hf) noexcept : hfunc(hf) {}
+    explicit func_wrapper_base(func_handle &&hf) noexcept : hfunc(std::move(hf)) {}
```

Author: rwgk

include/pybind11/functional.h

@@ -9,12 +9,55 @@
 
 #pragma once
 
+#define PYBIND11_HAS_TYPE_CASTER_STD_FUNCTION_SPECIALIZATIONS
+
 #include "pybind11.h"
 
 #include <functional>
 
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 PYBIND11_NAMESPACE_BEGIN(detail)
+PYBIND11_NAMESPACE_BEGIN(type_caster_std_function_specializations)
+
+// ensure GIL is held during functor destruction
+struct func_handle {
+    function f;
+#if !(defined(_MSC_VER) && _MSC_VER == 1916 && defined(PYBIND11_CPP17))
+    // This triggers a syntax error under very special conditions (very weird indeed).
+    explicit
+#endif
+        func_handle(function &&f_) noexcept
+        : f(std::move(f_)) {
+    }
+    func_handle(const func_handle &f_) { operator=(f_); }
+    func_handle &operator=(const func_handle &f_) {
+        gil_scoped_acquire acq;
+        f = f_.f;
+        return *this;
+    }
+    ~func_handle() {
+        gil_scoped_acquire acq;
+        function kill_f(std::move(f));
+    }
+};
+
+// to emulate 'move initialization capture' in C++11
+struct func_wrapper_base {
+    func_handle hfunc;
+    explicit func_wrapper_base(func_handle &&hf) noexcept : hfunc(std::move(hf)) {}
+};
+
+template <typename Return, typename... Args>
+struct func_wrapper : func_wrapper_base {
+    using func_wrapper_base::func_wrapper_base;
+    Return operator()(Args... args) const {
+        gil_scoped_acquire acq;
+        // casts the returned object as a rvalue to the return type
+        return hfunc.f(std::forward<Args>(args)...).template cast<Return>();
+    }
+};
+
+PYBIND11_NAMESPACE_END(type_caster_std_function_specializations)
 
 template <typename Return, typename... Args>
 struct type_caster<std::function<Return(Args...)>> {
@@ -77,40 +120,8 @@ struct type_caster<std::function<Return(Args...)>> {
             // See PR #1413 for full details
         }
 
-        // ensure GIL is held during functor destruction
-        struct func_handle {
-            function f;
-#if !(defined(_MSC_VER) && _MSC_VER == 1916 && defined(PYBIND11_CPP17))
-            // This triggers a syntax error under very special conditions (very weird indeed).
-            explicit
-#endif
-                func_handle(function &&f_) noexcept
-                : f(std::move(f_)) {
-            }
-            func_handle(const func_handle &f_) { operator=(f_); }
-            func_handle &operator=(const func_handle &f_) {
-                gil_scoped_acquire acq;
-                f = f_.f;
-                return *this;
-            }
-            ~func_handle() {
-                gil_scoped_acquire acq;
-                function kill_f(std::move(f));
-            }
-        };
-
-        // to emulate 'move initialization capture' in C++11
-        struct func_wrapper {
-            func_handle hfunc;
-            explicit func_wrapper(func_handle &&hf) noexcept : hfunc(std::move(hf)) {}
-            Return operator()(Args... args) const {
-                gil_scoped_acquire acq;
-                // casts the returned object as a rvalue to the return type
-                return hfunc.f(std::forward<Args>(args)...).template cast<Return>();
-            }
-        };
-
-        value = func_wrapper(func_handle(std::move(func)));
+        value = type_caster_std_function_specializations::func_wrapper<Return, Args...>(
+            type_caster_std_function_specializations::func_handle(std::move(func)));
         return true;
     }
 

tests/CMakeLists.txt

@@ -177,6 +177,7 @@ set(PYBIND11_TEST_FILES
     test_tagbased_polymorphic
     test_thread
     test_type_caster_pyobject_ptr
+    test_type_caster_std_function_specializations
     test_union
     test_unnamed_namespace_a
     test_unnamed_namespace_b

tests/test_type_caster_std_function_specializations.cpp

@@ -0,0 +1,46 @@
+#include <pybind11/functional.h>
+#include <pybind11/pybind11.h>
+
+#include "pybind11_tests.h"
+
+namespace py = pybind11;
+
+namespace {
+
+struct SpecialReturn {
+    int value = 99;
+};
+
+} // namespace
+
+namespace pybind11 {
+namespace detail {
+namespace type_caster_std_function_specializations {
+
+template <typename... Args>
+struct func_wrapper<SpecialReturn, Args...> : func_wrapper_base {
+    using func_wrapper_base::func_wrapper_base;
+    SpecialReturn operator()(Args... args) const {
+        gil_scoped_acquire acq;
+        SpecialReturn result;
+        try {
+            result = hfunc.f(std::forward<Args>(args)...).template cast<SpecialReturn>();
+        } catch (error_already_set &) {
+            result.value += 1;
+        }
+        result.value += 100;
+        return result;
+    }
+};
+
+} // namespace type_caster_std_function_specializations
+} // namespace detail
+} // namespace pybind11
+
+TEST_SUBMODULE(type_caster_std_function_specializations, m) {
+    py::class_<SpecialReturn>(m, "SpecialReturn")
+        .def(py::init<>())
+        .def_readwrite("value", &SpecialReturn::value);
+    m.def("call_callback_with_special_return",
+          [](const std::function<SpecialReturn()> &func) { return func(); });
+}

tests/test_type_caster_std_function_specializations.py

@@ -0,0 +1,15 @@
+from __future__ import annotations
+
+from pybind11_tests import type_caster_std_function_specializations as m
+
+
+def test_callback_with_special_return():
+    def return_special():
+        return m.SpecialReturn()
+
+    def raise_exception():
+        raise ValueError("called raise_exception.")
+
+    assert return_special().value == 99
+    assert m.call_callback_with_special_return(return_special).value == 199
+    assert m.call_callback_with_special_return(raise_exception).value == 200