PEP 613: Loosen class scope restriction of explicit TypeAlias (#2154)

Per discussion on typing-sig
https://mail.python.org/archives/list/[email protected]/thread/CGOO7GPPECGMLFDUDXSSXTRADI4BXYCS/

Co-authored-by: CAM Gerlach <[email protected]>

Author: nipunn1313

pep-0613.rst

@@ -95,6 +95,17 @@ across the codebase can be suppressed.
 Scope Restrictions:
 *******************
 
+::
+
+  class Foo:
+    x = ClassName
+    y: TypeAlias = ClassName
+    z: Type[ClassName] = ClassName
+
+Type aliases are valid within class scope, both implicitly (``x``) and
+explicitly (``y``). If the line should be interpreted as a class
+variable, it must be explicitly annotated (``z``).
+
 ::
 
   x = ClassName
@@ -103,7 +114,7 @@ Scope Restrictions:
 
 The outer ``x`` is a valid type alias, but type checkers must error if the
 inner ``x`` is ever used as a type because type aliases cannot be defined
-inside a nested scope.
+inside of a function.
 This is confusing because the alias declaration rule is not explicit, and because
 a type error will not be thrown on the location of the inner type alias declaration
 but rather on every one of its subsequent use cases.
@@ -116,10 +127,10 @@ but rather on every one of its subsequent use cases.
   def bar() -> None:
     x: TypeAlias = ClassName
 
-With explicit aliases, the outer assignment is still a valid type variable,
-and the inner assignment can either be a valid local variable or a clear error,
-communicating to the author that type aliases cannot be defined inside a nested
-scope.
+With explicit aliases, the outer assignment is still a valid type variable.
+Inside ``foo``, the inner assignment should be interpreted as ``x: Type[ClassName]``.
+Inside ``bar``, the type checker should raise a clear error, communicating
+to the author that type aliases cannot be defined inside a function.
 
 
 Specification
@@ -200,6 +211,14 @@ appealing because it still sticks with the ``MyType = int`` assignment
 syntax, and adds some information for the type checker purely as an annotation.
 
 
+Version History
+===============
+
+* 2021-11-16
+
+    * Allow TypeAlias inside class scope
+
+
 Copyright
 =========