add argument number dispatch mechanism for std::function casting

include/pybind11/functional.h

@@ -101,8 +101,17 @@ public:
                 if (detail::is_function_record_capsule(c)) {
                     rec = c.get_pointer<function_record>();
                 }
-
                 while (rec != nullptr) {
+                    const int correctingSelfArgument = rec->is_method ? 1 : 0;
+                    if (rec->nargs - correctingSelfArgument != sizeof...(Args)) {
+                        rec = rec->next;
+                        // if the overload is not feasible in terms of number of arguments, we
+                        // continue to the next one. If there is no next one, we return false.
+                        if (rec == nullptr) {
+                            return false;
+                        }
+                        continue;
+                    }
                     if (rec->is_stateless
                         && same_type(typeid(function_type),
                                      *reinterpret_cast<const std::type_info *>(rec->data[1]))) {
@@ -118,6 +127,36 @@ public:
             // PYPY segfaults here when passing builtin function like sum.
             // Raising an fail exception here works to prevent the segfault, but only on gcc.
             // See PR #1413 for full details
+        } else {
+            // Check number of arguments of Python function
+            auto getArgCount = [&](PyObject *obj) {
+                // This is faster then doing import inspect and inspect.signature(obj).parameters
+                auto *t = PyObject_GetAttrString(obj, "__code__");
+                auto *argCount = PyObject_GetAttrString(t, "co_argcount");
+                return PyLong_AsLong(argCount);
+            };
+            long argCount = -1;
+
+            if (static_cast<bool>(PyObject_HasAttrString(src.ptr(), "__code__"))) {
+                argCount = getArgCount(src.ptr());
+            } else {
+                if (static_cast<bool>(PyObject_HasAttrString(src.ptr(), "__call__"))) {
+                    auto *t2 = PyObject_GetAttrString(src.ptr(), "__call__");
+                    argCount = getArgCount(t2) - 1; // we have to remove the self argument
+                } else {
+                    // No __code__ or __call__ attribute, this is not a proper Python function
+                    return false;
+                }
+            }
+            // if we are a method, we have to correct the argument count since we are not counting
+            // the self argument
+            const int correctingSelfArgument
+                = static_cast<bool>(PyMethod_Check(src.ptr())) ? 1 : 0;
+
+            argCount -= correctingSelfArgument;
+            if (argCount != sizeof...(Args)) {
+                return false;
+            }
         }
 
         value = type_caster_std_function_specializations::func_wrapper<Return, Args...>(

tests/test_callbacks.cpp

@@ -170,6 +170,12 @@ TEST_SUBMODULE(callbacks, m) {
         return "argument does NOT match dummy_function. This should never happen!";
     });
 
+    // test_cpp_correct_overload_resolution
+    m.def("dummy_function_overloaded_std_func_arg",
+          [](const std::function<int(int)> &f) { return 3 * f(3); });
+    m.def("dummy_function_overloaded_std_func_arg",
+          [](const std::function<int(int, int)> &f) { return 2 * f(3, 4); });
+
     class AbstractBase {
     public:
         // [workaround(intel)] = default does not work here

tests/test_callbacks.py

@@ -103,6 +103,20 @@ def test_cpp_callable_cleanup():
     assert alive_counts == [0, 1, 2, 1, 2, 1, 0]
 
 
+def test_cpp_correct_overload_resolution():
+    def f(a):
+        return a
+
+    assert m.dummy_function_overloaded_std_func_arg(f) == 9
+    assert m.dummy_function_overloaded_std_func_arg(lambda i: i) == 9
+
+    def f2(a, b):
+        return a + b
+
+    assert m.dummy_function_overloaded_std_func_arg(f2) == 14
+    assert m.dummy_function_overloaded_std_func_arg(lambda i, j: i + j) == 14
+
+
 def test_cpp_function_roundtrip():
     """Test if passing a function pointer from C++ -> Python -> C++ yields the original pointer"""
 
@@ -131,7 +145,10 @@ def test_cpp_function_roundtrip():
         m.test_dummy_function(lambda x, y: x + y)
     assert any(
         s in str(excinfo.value)
-        for s in ("missing 1 required positional argument", "takes exactly 2 arguments")
+        for s in (
+            "incompatible function arguments. The following argument types are",
+            "function test_cpp_function_roundtrip.<locals>.<lambda>",
+        )
     )
 
 

tests/test_embed/test_interpreter.cpp

@@ -1,4 +1,5 @@
 #include <pybind11/embed.h>
+#include <pybind11/functional.h>
 
 // Silence MSVC C++17 deprecation warning from Catch regarding std::uncaught_exceptions (up to
 // catch 2.0.1; this should be fixed in the next catch release after 2.0.1).
@@ -78,6 +79,12 @@ PYBIND11_EMBEDDED_MODULE(throw_error_already_set, ) {
     d["missing"].cast<py::object>();
 }
 
+PYBIND11_EMBEDDED_MODULE(func_module, m) {
+    m.def("funcOverload", [](const std::function<int(int, int)> &f) {
+         return f(2, 3);
+     }).def("funcOverload", [](const std::function<int(int)> &f) { return f(2); });
+}
+
 TEST_CASE("PYTHONPATH is used to update sys.path") {
     // The setup for this TEST_CASE is in catch.cpp!
     auto sys_path = py::str(py::module_::import("sys").attr("path")).cast<std::string>();
@@ -171,6 +178,15 @@ TEST_CASE("There can be only one interpreter") {
     py::initialize_interpreter();
 }
 
+TEST_CASE("Check the overload resolution from cpp_function objects to std::function") {
+    auto m = py::module_::import("func_module");
+    auto f = std::function<int(int)>([](int x) { return 2 * x; });
+    REQUIRE(m.attr("funcOverload")(f).template cast<int>() == 4);
+
+    auto f2 = std::function<int(int, int)>([](int x, int y) { return 2 * x * y; });
+    REQUIRE(m.attr("funcOverload")(f2).template cast<int>() == 12);
+}
+
 #if PY_VERSION_HEX >= PYBIND11_PYCONFIG_SUPPORT_PY_VERSION_HEX
 TEST_CASE("Custom PyConfig") {
     py::finalize_interpreter();