Mypy should warn when comparing lists that hold a union of incomparable types

Feature

As of mypy 0.991 on Python 3.11.0, mypy --strict doesn't report any problems on this code:

from typing import List, Union
a: List[Union[int, str]] = ['a']
b: List[Union[int, str]] = [1]
result = a < b

Despite mypy passing, actually running this results in TypeError: '<' not supported between instances of 'str' and 'int'. I think ideally mypy would catch this.

Pitch

While the problem was much less obvious, this affected a real project at https://github.com/certbot/certbot/issues/9481 so I think it'd be useful if mypy could help stop bugs like this from slipping through.

My knowledge of mypy and especially its internals is pretty limited, but is there a sane way to do this? Would it be possible to do something like write the annotation of list.__lt__ in the typeshed so it only accepts another list whose elements are of a type that can be compared with its own?

Inizia riproducendo l’esempio segnalato in Python 3.11 con mypy --strict e conferma il TypeError a runtime. Analizza come è tipizzato list.lt e se typeshed può esprimere tipi di elementi confrontabili. Il lavoro è completo quando mypy segnala il confronto non sicuro senza rifiutare confronti validi tra liste confrontabili.