diff --git a/sphinx/ext/autodoc/directive.py b/sphinx/ext/autodoc/directive.py
index dfac5e0e6..32adc342d 100644
--- a/sphinx/ext/autodoc/directive.py
+++ b/sphinx/ext/autodoc/directive.py
@@ -87,7 +87,8 @@ def process_documenter_options(documenter: "Type[Documenter]", config: Config, o
             negated = options.pop('no-' + name, True) is None
             if name in config.autodoc_default_options and not negated:
                 if name in options:
-                    # take value from options if present or extend it with autodoc_default_options if necessary
+                    # take value from options if present or extend it
+                    # with autodoc_default_options if necessary
                     if name in AUTODOC_EXTENDABLE_OPTIONS:
                         if options[name] is not None and options[name].startswith('+'):
                             options[name] = ','.join([config.autodoc_default_options[name],
diff --git a/tests/test_ext_autodoc.py b/tests/test_ext_autodoc.py
index 57401133f..bad8da61d 100644
--- a/tests/test_ext_autodoc.py
+++ b/tests/test_ext_autodoc.py
@@ -835,6 +835,10 @@ def test_autodoc_special_members(app):
         '   .. py:method:: Class.undocmeth()'
     ]
 
+
+@pytest.mark.sphinx('html', testroot='ext-autodoc')
+def test_autodoc_special_members_extentiable(app):
+
     # specific special methods from autodoc_default_options
     options = {"undoc-members": None}
     app.config.autodoc_default_options["special-members"] = "__special2__"