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.

Lib/test/test_typing.py

@@ -24,6 +24,7 @@ from typing import NamedTuple, TypedDict
 from typing import IO, TextIO, BinaryIO
 from typing import Pattern, Match
 from typing import Annotated, ForwardRef
+from typing import TypeAlias
 import abc
 import typing
 import weakref
@@ -4176,6 +4177,45 @@ class AnnotatedTests(BaseTestCase):
         self.assertEqual(X[int], List[Annotated[int, 5]])
 
 
+class TypeAliasTests(BaseTestCase):
+    def test_canonical_usage_with_variable_annotation(self):
+        Alias: TypeAlias = Employee
+
+    def test_canonical_usage_with_type_comment(self):
+        Alias = Employee  # type: TypeAlias
+
+    def test_cannot_instantiate(self):
+        with self.assertRaises(TypeError):
+            TypeAlias()
+
+    def test_no_isinstance(self):
+        with self.assertRaises(TypeError):
+            isinstance(42, TypeAlias)
+
+    def test_no_issubclass(self):
+        with self.assertRaises(TypeError):
+            issubclass(Employee, TypeAlias)
+
+        with self.assertRaises(TypeError):
+            issubclass(TypeAlias, Employee)
+
+    def test_cannot_subclass(self):
+        with self.assertRaises(TypeError):
+            class C(TypeAlias):
+                pass
+
+        with self.assertRaises(TypeError):
+            class C(type(TypeAlias)):
+                pass
+
+    def test_repr(self):
+        self.assertEqual(repr(TypeAlias), 'typing.TypeAlias')
+
+    def test_cannot_subscript(self):
+        with self.assertRaises(TypeError):
+            TypeAlias[int]
+
+
 class AllTests(BaseTestCase):
     """Tests for __all__."""