Allow NULL value in pybind11_meta_setattro (#2629)

include/pybind11/detail/class.h

@@ -129,7 +129,7 @@ extern "C" inline int pybind11_meta_setattro(PyObject* obj, PyObject* name, PyOb
     //   2. `Type.static_prop = other_static_prop` --> setattro:  replace existing `static_prop`
     //   3. `Type.regular_attribute = value`       --> setattro:  regular attribute assignment
     const auto static_prop = (PyObject *) get_internals().static_property_type;
-    const auto call_descr_set = descr && PyObject_IsInstance(descr, static_prop)
+    const auto call_descr_set = descr && value && PyObject_IsInstance(descr, static_prop)
                                 && !PyObject_IsInstance(value, static_prop);
     if (call_descr_set) {
         // Call `static_property.__set__()` instead of replacing the `static_property`.

tests/test_methods_and_attributes.py

@@ -171,6 +171,19 @@ def test_static_properties():
     assert m.TestPropertiesOverride().def_readonly == 99
     assert m.TestPropertiesOverride.def_readonly_static == 99
 
+    # Only static attributes can be deleted
+    del m.TestPropertiesOverride.def_readonly_static
+    assert (
+        hasattr(m.TestPropertiesOverride, "def_readonly_static")
+        and m.TestPropertiesOverride.def_readonly_static
+        is m.TestProperties.def_readonly_static
+    )
+    assert "def_readonly_static" not in m.TestPropertiesOverride.__dict__
+    properties_override = m.TestPropertiesOverride()
+    with pytest.raises(AttributeError) as excinfo:
+        del properties_override.def_readonly
+    assert "can't delete attribute" in str(excinfo.value)
+
 
 def test_static_cls():
     """Static property getter and setters expect the type object as the their only argument"""