bpo-42195: Override _CallableGenericAlias's __getitem__ (GH-23915)

Added `__getitem__` for `_CallableGenericAlias` so that it returns a subclass (itself) of `types.GenericAlias` rather than the default behavior of returning a plain `types.GenericAlias`. This fixes `repr` issues occuring after `TypeVar` substitution arising from the previous behavior.

1 files changed, 6 insertions, 0 deletions

diff --git a/Lib/test/test_genericalias.py b/Lib/test/test_genericalias.py
index 5de13fe..ccf40b1 100644
--- a/Lib/test/test_genericalias.py
+++ b/Lib/test/test_genericalias.py
@@ -347,6 +347,12 @@ class BaseTest(unittest.TestCase):
             self.assertEqual(C2[int, float, str], Callable[[int, float], str])
             self.assertEqual(C3[int], Callable[..., int])
 
+            # multi chaining
+            C4 = C2[int, V, str]
+            self.assertEqual(repr(C4).split(".")[-1], "Callable[[int, ~V], str]")
+            self.assertEqual(repr(C4[dict]).split(".")[-1], "Callable[[int, dict], str]")
+            self.assertEqual(C4[dict], Callable[[int, dict], str])
+
         with self.subTest("Testing type erasure"):
             class C1(Callable):
                 def __call__(self):