Type narrowing for subscripting is too eager

[JelleZijlstra]

import pandas as pd


def x():

    res = pd.DataFrame([1, 2, 3])

    res["mean"] = [None] * 3
    reveal_type(res["mean"])

This will currently produce something like list[None], but that's incorrect because DataFrames return Series objects from __getitem__. Mypy and pyright get this right, so it may be worth looking into what heuristics they are using.

Solution I can think of: currently NameCheckVisitor.composite_from_subscript sets the narrowed type, regardless of the type of the subscripted object. If we instead did this in list.__setitem__ and dict._setitem__ (where we know it's safe), that would solve the problem.