Add back _AnnotatedAlias (python#587)

Fixes python#586

Author: JelleZijlstra

src/test_typing_extensions.py

@@ -5251,6 +5251,11 @@ def test_nested_annotated_with_unhashable_metadata(self):
         self.assertEqual(X.__origin__, List[Annotated[str, {"unhashable_metadata"}]])
         self.assertEqual(X.__metadata__, ("metadata",))
 
+    def test_compatibility(self):
+        # Test that the _AnnotatedAlias compatibility alias works
+        self.assertTrue(hasattr(typing_extensions, "_AnnotatedAlias"))
+        self.assertIs(typing_extensions._AnnotatedAlias, typing._AnnotatedAlias)
+
 
 class GetTypeHintsTests(BaseTestCase):
     def test_get_type_hints(self):

src/typing_extensions.py

@@ -4095,7 +4095,7 @@ def evaluate_forward_ref(
         )
 
 
-# Aliases for items that have always been in typing.
+# Aliases for items that are in typing in all supported versions.
 # Explicitly assign these (rather than using `from typing import *` at the top),
 # so that we get a CI error if one of these is deleted from typing.py
 # in a future version of Python
@@ -4136,3 +4136,6 @@ def evaluate_forward_ref(
 cast = typing.cast
 no_type_check = typing.no_type_check
 no_type_check_decorator = typing.no_type_check_decorator
+# This is private, but it was defined by typing_extensions for a long time
+# and some users rely on it.
+_AnnotatedAlias = typing._AnnotatedAlias