During metaclass.__init__, super() of the constructed class does not work

Doc/whatsnew/3.6.rst

 ****************************
   What's New In Python 3.6
 ****************************
 
 :Release: |release|
 :Date: |today|
 :Editors: Elvis Pranskevichus <elvis@magic.io>, Yury Selivanov <yury@magic.io>
 
 .. Rules for maintenance:
 
@@ skipping 332 matching lines @@
 
         def __init_subclass__(cls, **kwargs):
             super().__init_subclass__(**kwargs)
             cls.subclasses.append(cls)
 
     class Plugin1(PluginBase):
         pass
 
     class Plugin2(PluginBase):
         pass
+
+In order to allow zero-argument :func:`super` calls to work correctly from
+``__init_subclass__`` implementations, custom metaclasses must ensure that
+the new ``__classcell__`` namespace entry is propagated to ``type.__new__``.
 
 .. seealso::
 
  :pep:`487` -- Simpler customization of class creation
     PEP written and implemented by Martin Teichmann.
 
  :ref:`Feature documentation <class-customization>`
 
 
 .. _whatsnew36-pep487-descriptors:
@@ skipping 1865 matching lines @@
   now includes the group index and the position of the reference.
   (Contributed by SilentGhost, Serhiy Storchaka in :issue:`25953`.)
 
 * :class:`zipfile.ZipFile` will now raise :exc:`NotImplementedError` for
   unrecognized compression values.  Previously a plain :exc:`RuntimeError`
   was raised.  Additionally, calling :class:`~zipfile.ZipFile` methods
   on a closed ZipFile or calling the :meth:`~zipfile.ZipFile.write` method
   on a ZipFile created with mode ``'r'`` will raise a :exc:`ValueError`.
   Previously, a :exc:`RuntimeError` was raised in those scenarios.
 
+* when custom metaclasses are combined with zero-argument :func:`super` or
+  direct references from methods to the implicit ``__class__`` closure
+  variable, the implicit ``__classcell__`` namespace entry must now be passed
+  up to ``type.__new__`` for initialisation. Failing to do so will result in
+  a :exc:`DeprecationWarning` in 3.6 and a :exc:`RuntimeWarning` in the future.
 
 Changes in the C API
 --------------------
 
 * The :c:func:`PyMem_Malloc` allocator family now uses the :ref:`pymalloc allocator
   <pymalloc>` rather than the system :c:func:`malloc`. Applications calling
   :c:func:`PyMem_Malloc` without holding the GIL can now crash. Set the
   :envvar:`PYTHONMALLOC` environment variable to ``debug`` to validate the
   usage of memory allocators in your application. See :issue:`26249`.
 
@@ skipping 26 matching lines @@
   have been modified, the new :opcode:`CALL_FUNCTION_EX` and
   :opcode:`BUILD_TUPLE_UNPACK_WITH_CALL` have been added, and
   ``CALL_FUNCTION_VAR``, ``CALL_FUNCTION_VAR_KW`` and ``MAKE_CLOSURE`` opcodes
   have been removed.
   (Contributed by Demur Rumed in :issue:`27095`, and Serhiy Storchaka in
   :issue:`27213`, :issue:`28257`.)
 
 * The new :opcode:`SETUP_ANNOTATIONS` and :opcode:`STORE_ANNOTATION` opcodes
   have been added to support the new :term:`variable annotation` syntax.
   (Contributed by Ivan Levkivskyi in :issue:`27985`.)