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?

Start with the example in mypy/suggestions.py around lines 509-527 and reproduce the possibly-undefined warning with the option enabled. Trace the type-checking path for the assert arg_types example; done when this assertion correctly establishes that the sequence is non-empty and removes the warning.