Conditional import

Author: Tinche

src/cattrs/v/_fluent.py

@@ -12,9 +12,13 @@
     Sequence,
     Sized,
     TypeVar,
-    assert_never,
 )
 
+try:
+    from typing import assert_never
+except ImportError:
+    from typing_extensions import assert_never
+
 from attrs import Attribute, AttrsInstance, define
 from attrs import fields as f
 
@@ -279,7 +283,7 @@ def customize(
             )
         else:
             # The match is exhaustive.
-            assert_never()
+            assert_never(field)
     res = make_dict_structure_fn(
         cl,
         converter,