Support positional-only parameters in classmethods (#11635)

CHANGES

@@ -5,6 +5,8 @@ Dependencies
 ------------
 
 * #11576: Require sphinxcontrib-serializinghtml 1.1.9.
+* #11543: autodoc: Support positional-only parameters in ``classmethod`` methods
+  when ``autodoc_preserve_defaults`` is ``True``.
 
 Bugs fixed
 ----------

sphinx/ext/autodoc/preserve_defaults.py

@@ -135,13 +135,21 @@ def update_defvalue(app: Sphinx, obj: Any, bound_method: bool) -> None:
 
     try:
         args = _get_arguments(obj)
+    except SyntaxError:
+        return
     if args is None:
         # If the object is a built-in, we won't be always able to recover
         # the function definition and its arguments. This happens if *obj*
         # is the `__init__` method generated automatically for dataclasses.
         return
 
-        if args.defaults or args.kw_defaults:
+    if not args.defaults and not args.kw_defaults:
+        return
+
+    try:
+        if bound_method and inspect.ismethod(obj) and hasattr(obj, '__func__'):
+            sig = inspect.signature(obj.__func__)
+        else:
             sig = inspect.signature(obj)
         defaults = list(args.defaults)
         kw_defaults = list(args.kw_defaults)
@@ -165,21 +173,19 @@ def update_defvalue(app: Sphinx, obj: Any, bound_method: bool) -> None:
                         value = ast_unparse(default)
                     parameters[i] = param.replace(default=DefaultValue(value))
 
-            if bound_method and inspect.ismethod(obj):
-                # classmethods
-                cls = inspect.Parameter('cls', inspect.Parameter.POSITIONAL_OR_KEYWORD)
-                parameters.insert(0, cls)
-
         sig = sig.replace(parameters=parameters)
-            if bound_method and inspect.ismethod(obj):
-                # classmethods can't be assigned __signature__ attribute.
-                obj.__dict__['__signature__'] = sig
-            else:
+        try:
             obj.__signature__ = sig
+        except AttributeError:
+            # __signature__ can't be set directly on bound methods.
+            obj.__dict__['__signature__'] = sig
     except (AttributeError, TypeError):
-        # failed to update signature (ex. built-in or extension types)
-        pass
-    except NotImplementedError as exc:  # failed to ast.unparse()
+        # Failed to update signature (e.g. built-in or extension types).
+        # For user-defined functions, "obj" may not have __dict__,
+        # e.g. when decorated with a class that defines __slots__.
+        # In this case, we can't set __signature__.
+        return
+    except NotImplementedError as exc:  # failed to ast_unparse()
         logger.warning(__("Failed to parse a default argument value for %r: %s"), obj, exc)