Type-narrowing of a list item for an inline list multiplied with a type-guard on said item

Feature

I would like the following to work:

from typing import assert_type

def _(foo: str | None) -> None:
    assert_type(["a", "b", foo] * bool(foo), list[str])

playground url: https://mypy-play.net/?gist=f6454b1662920bc8df00dfe047a80044

To my understanding, this would require 2 things that mypy currently doesn't do as shown by the above playground:

1. Understanding bool is a truthyness typeguard. ie roughly treating bool(a) as: def bool[T](a: T | |Literal['', False, 0] | None) -> TypeGuard[T]: ...
2. Doing type-narrowing on list.__mul__ with a type-guard.

Pitch

Let me quote @jaraco directly, as this is a pattern used a lot in his ecosystem (which includes setuptools): https://github.com/jaraco/jaraco.context/pull/16#discussion_r2967882086

I very much prefer to use algebraic expressions over branching logic. The expressiveness of multiplying by a bool is important to the aesthetic of simplicity here.

Inizia con il mypy playground collegato e riproduci l’esempio assert_type. Analizza i due comportamenti richiesti—il restringimento basato sulla truthiness tramite bool(foo) e il restringimento del tipo degli elementi di una lista inline moltiplicata per quel guard—e considera completata la issue quando l’esempio viene accettato con list[str] come asserzione.