Type narrowing issue: bounded TypeVar loses type information when list is sliced

Bug Report

When slicing a list variable that is annotated with a bounded list TypeVar, mypy reveals the type of the sliced list as builtins.list[Any] instead of retaining the bounded type variable T. This results in a loss of type information.

To Reproduce

import typing

T = typing.TypeVar("T", bound=list)

def example(a: T):
    reveal_type(a[:1])  # note: Revealed type is "builtins.list[Any]"

In the above code, I would expect the revealed type of a[:1] to be T, representing a sublist of the input list a. However, mypy instead reveals the type as builtins.list[Any], indicating a loss of the bounded type information.

Expected Behavior

I expected the revealed type to retain the bounded type variable T, which would maintain the type information of the elements in the list.

Actual Behavior

The revealed type is builtins.list[Any], demonstrating a loss of type information.

Your Environment

• Mypy version used: 1.5.0
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): not relevent i guess
• Python version used: 3.9

Inizia eseguendo il riproduttore fornito per Python 3.9 con mypy 1.5.0 e tracciando come vengono analizzati i TypeVars vincolati e lo slicing delle liste. Confronta il tipo rivelato con il T previsto, quindi aggiungi la copertura di regressione per questo esempio e verifica che le informazioni sul tipo vincolato vengano mantenute.