bpo-42740: Fix get_args for PEP 585 collections.abc.Callable (GH-23963)

PR 1/2. Needs backport to 3.9.
(cherry picked from commit 4140f10)

Co-authored-by: Ken Jin <[email protected]>

Author: miss-islington

Lib/test/test_typing.py

@@ -3071,6 +3071,11 @@ class C(Generic[T]): pass
         self.assertEqual(get_args(Callable), ())
         self.assertEqual(get_args(list[int]), (int,))
         self.assertEqual(get_args(list), ())
+        self.assertEqual(get_args(collections.abc.Callable[[int], str]), ([int], str))
+        self.assertEqual(get_args(collections.abc.Callable[..., str]), (..., str))
+        self.assertEqual(get_args(collections.abc.Callable[[], str]), ([], str))
+        self.assertEqual(get_args(collections.abc.Callable[[int], str]),
+                         get_args(Callable[[int], str]))
 
 
 class CollectionsAbcTests(BaseTestCase):

Lib/typing.py

@@ -1507,13 +1507,11 @@ def get_args(tp):
     """
     if isinstance(tp, _AnnotatedAlias):
         return (tp.__origin__,) + tp.__metadata__
-    if isinstance(tp, _GenericAlias):
+    if isinstance(tp, (_GenericAlias, GenericAlias)):
         res = tp.__args__
         if tp.__origin__ is collections.abc.Callable and res[0] is not Ellipsis:
             res = (list(res[:-1]), res[-1])
         return res
-    if isinstance(tp, GenericAlias):
-        return tp.__args__
     return ()