Possibly-undefined: allow assertions on the lengths of sequences

The new (disabled-by-default) possibly-undefined error code is great, and I'm using it in several of my projects. There are many hits if you enable it for typechecking mypy's own code, however. An example is this function, where mypy (correctly) points out that arg may be undefined on line 526 (arg_types might be an empty list).

https://github.com/python/mypy/blob/456dcbdd6a2f9b36249e0e7d31e521f5678a8235/mypy/suggestions.py#L509-L527

It would be nice if we could use assertions based on the truthiness of a sequence to make this error go away. E.g., it would be nice if we could silence the error here with this diff:

--- a/mypy/suggestions.py
+++ b/mypy/suggestions.py
@@ -512,6 +512,7 @@ class SuggestionEngine:
         arg_names: list[list[str | None]],
         arg_types: list[list[Type]],
     ) -> str:
+        assert arg_types
         args: list[str] = []

Mypy doesn't currently understand this idiom, however. @ilinum, how hard might this kind of thing be to implement?

Commencez par l’exemple dans mypy/suggestions.py autour des lignes 509-527 et reproduisez l’avertissement possibly-undefined avec l’option activée. Suivez le chemin de vérification des types pour l’exemple assert arg_types ; c’est terminé lorsque cette assertion établit correctement que la séquence n’est pas vide et supprime l’avertissement.