fix: enable py::implicitly_convertible<py::none, ...> for py::class_-wrapped types (#3059)

* Allow casting from None to a custom object, closes #2778

* ci.yml patch from the smart_holder branch for full CI coverage.

include/pybind11/detail/type_caster_base.h

@@ -657,12 +657,6 @@ public:
     PYBIND11_NOINLINE bool load_impl(handle src, bool convert) {
         if (!src) return false;
         if (!typeinfo) return try_load_foreign_module_local(src);
-        if (src.is_none()) {
-            // Defer accepting None to other overloads (if we aren't in convert mode):
-            if (!convert) return false;
-            value = nullptr;
-            return true;
-        }
 
         auto &this_ = static_cast<ThisT &>(*this);
         this_.check_holder_compat();
@@ -731,7 +725,19 @@ public:
         }
 
         // Global typeinfo has precedence over foreign module_local
-        return try_load_foreign_module_local(src);
+        if (try_load_foreign_module_local(src)) {
+           return true;
+        }
+
+        // Custom converters didn't take None, now we convert None to nullptr.
+        if (src.is_none()) {
+           // Defer accepting None to other overloads (if we aren't in convert mode):
+           if (!convert) return false;
+           value = nullptr;
+           return true;
+        }
+
+        return false;
     }
 
 

tests/test_methods_and_attributes.cpp

@@ -118,6 +118,14 @@ int none3(const std::shared_ptr<NoneTester> &obj) { return obj ? obj->answer : -
 int none4(std::shared_ptr<NoneTester> *obj) { return obj && *obj ? (*obj)->answer : -1; }
 int none5(const std::shared_ptr<NoneTester> &obj) { return obj ? obj->answer : -1; }
 
+// Issue #2778: implicit casting from None to object (not pointer)
+class NoneCastTester {
+public:
+    int answer = -1;
+    NoneCastTester() = default;
+    NoneCastTester(int v) : answer(v) {};
+};
+
 struct StrIssue {
     int val = -1;
 
@@ -358,6 +366,16 @@ TEST_SUBMODULE(methods_and_attributes, m) {
     m.def("no_none_kwarg", &none2, "a"_a.none(false));
     m.def("no_none_kwarg_kw_only", &none2, py::kw_only(), "a"_a.none(false));
 
+    // test_casts_none
+    // Issue #2778: implicit casting from None to object (not pointer)
+    py::class_<NoneCastTester>(m, "NoneCastTester")
+          .def(py::init<>())
+          .def(py::init<int>())
+          .def(py::init([](py::none const&) { return NoneCastTester{}; }));
+    py::implicitly_convertible<py::none, NoneCastTester>();
+    m.def("ok_obj_or_none", [](NoneCastTester const& foo) { return foo.answer; });
+
+
     // test_str_issue
     // Issue #283: __str__ called on uninitialized instance when constructor arguments invalid
     py::class_<StrIssue>(m, "StrIssue")

tests/test_methods_and_attributes.py

@@ -433,6 +433,17 @@ def test_accepts_none(msg):
         assert m.ok_none4(None) == -1
 
 
+def test_casts_none(msg):
+    """#2778: implicit casting from None to object (not pointer)"""
+    a = m.NoneCastTester()
+    assert m.ok_obj_or_none(a) == -1
+    a = m.NoneCastTester(4)
+    assert m.ok_obj_or_none(a) == 4
+    a = m.NoneCastTester(None)
+    assert m.ok_obj_or_none(a) == -1
+    assert m.ok_obj_or_none(None) == -1
+
+
 def test_str_issue(msg):
     """#283: __str__ called on uninitialized instance when constructor arguments invalid"""