feature request/idea: tuple length inference from assertions/tests on len()

[aecay]

Consider this code:

from typing import List, Any, Tuple

def fn_returning_sequence() -> List[Any]:
    return [1, 2, 3]

x = tuple(fn_returning_sequence())
assert len(x) == 3
reveal_type(x)

It would be nice if mypy's type inference was extended so that it knew that x was a Tuple[Any, Any, Any] rather than the less specific Tuple[Any, ...]. And similarly for cases like if len(x) == 2: ...; in the body of the if mypy can be sure that x is a Tuple[Any, Any].

(Confusingly, the actual value printed by reveal_type in the example is builtins.tuple[Any], but as far as I can tell this is a synonym of Tuple[Any, ...])

[hauntsaninja]

Similar to #7509 and #8733

[JukkaL]

Closing as duplicate of #8733.