PEP 827: Make UpdateClass take the class to update as an argument

peps/pep-0827.rst

@@ -823,13 +823,17 @@ Raise error
 Update class
 ''''''''''''
 
-* ``UpdateClass[*Ps: Member]``: A special form that *updates* an
+* ``UpdateClass[Cls, *Ps: Member]``: A special form that *updates* an
   existing nominal class with new members (possibly overriding old
   ones, or removing them by making them have type ``Never``).
 
   This can only be used in the return type of a type decorator
   or as the return type of ``__init_subclass__``.
 
+  ``Cls`` must be the class to update. For ``__init_subclass__``,
+  ``cls`` ought to have ``type[T]`` and ``T`` should be passed as
+  ``Cls``.
+
   When a class is declared, if one or more of its ancestors have an
   ``__init_subclass__`` with an ``UpdateClass`` return type, they are
   applied in reverse MRO order. If the ``cls`` param is
@@ -1107,6 +1111,7 @@ based on iterating over all attributes.
     def dataclass_ish[T](
         cls: type[T],
     ) -> typing.UpdateClass[
+        T,
         # Add the computed __init__ function
         InitFnType[T],
     ]:
@@ -1120,6 +1125,7 @@ Or to create a base class (a la Pydantic) that does.
         def __init_subclass__[T](
             cls: type[T],
         ) -> typing.UpdateClass[
+            T,
             # Add the computed __init__ function
             InitFnType[T],
         ]: