bpo-41923: PEP 613: Add TypeAlias to typing module (#22532)

This special marker annotation is intended to help in distinguishing
proper PEP 484-compliant type aliases from regular top-level variable
assignments.

1 files changed, 23 insertions, 2 deletions

diff --git a/Doc/whatsnew/3.10.rst b/Doc/whatsnew/3.10.rst
index 2bcdba6..4ada4be 100644
--- a/Doc/whatsnew/3.10.rst
+++ b/Doc/whatsnew/3.10.rst
@@ -99,8 +99,29 @@ in :issue:`38605`.)
 * :pep:`618`: The :func:`zip` function now has an optional ``strict`` flag, used
   to require that all the iterables have an equal length.
 
-PEP604: New Type Operator
--------------------------
+PEP 613: TypeAlias Annotation
+-----------------------------
+
+:pep:`484` introduced the concept of type aliases, only requiring them to be
+top-level unannotated assignments. This simplicity sometimes made it difficult
+for type checkers to distinguish between type aliases and ordinary assignments,
+especially when forward references or invalid types were involved. Compare::
+
+   StrCache = 'Cache[str]'  # a type alias
+   LOG_PREFIX = 'LOG[DEBUG]'  # a module constant
+
+Now the :mod:`typing` module has a special annotation :data:`TypeAlias` to
+declare type aliases more explicitly::
+
+   StrCache: TypeAlias = 'Cache[str]'  # a type alias
+   LOG_PREFIX = 'LOG[DEBUG]'  # a module constant
+
+See :pep:`613` for more details.
+
+(Contributed by Mikhail Golubev in :issue:`41923`.)
+
+PEP604: New Type Union Operator
+-------------------------------
 
 A new type union operator was introduced which enables the syntax ``X | Y``.
 This provides a cleaner way of expressing 'either type X or type Y' instead of