bpo-23722: Raise a RuntimeError for absent __classcell__. (GH-6931)

A DeprecationWarning was emitted in Python 3.6-3.7.

2 files changed, 6 insertions, 2 deletions

diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst
index cc8dc95..855f241 100644
--- a/Doc/reference/datamodel.rst
+++ b/Doc/reference/datamodel.rst
@@ -1967,8 +1967,7 @@ current call is identified based on the first argument passed to the method.
    as a ``__classcell__`` entry in the class namespace. If present, this must
    be propagated up to the ``type.__new__`` call in order for the class to be
    initialised correctly.
-   Failing to do so will result in a :exc:`DeprecationWarning` in Python 3.6,
-   and a :exc:`RuntimeError` in Python 3.8.
+   Failing to do so will result in a :exc:`RuntimeError` in Python 3.8.
 
 When using the default metaclass :class:`type`, or any metaclass that ultimately
 calls ``type.__new__``, the following additional customisation steps are
diff --git a/Doc/whatsnew/3.8.rst b/Doc/whatsnew/3.8.rst
index 11538e2..9aad908 100644
--- a/Doc/whatsnew/3.8.rst
+++ b/Doc/whatsnew/3.8.rst
@@ -146,6 +146,11 @@ Changes in the Python API
   a database if it does not exist.
   (Contributed by Serhiy Storchaka in :issue:`32749`.)
 
+* A :exc:`RuntimeError` is now raised when the custom metaclass doesn't
+  provide the ``__classcell__`` entry in the namespace passed to
+  ``type.__new__``.  A :exc:`DeprecationWarning` was emitted in Python
+  3.6--3.7.  (Contributed by Serhiy Storchaka in :issue:`23722`.)
+
 
 CPython bytecode changes
 ------------------------