Feature proposal: Warn when narrowing to a mutable type

Feature

I often encourage people to use "immutable" types for function parameters, as a way to indicate that the passed data structures won't be mutated by the function. Of course that isn't a guarantee, but a nice indication that can make it easier to reason about code like this:

vals = [1, 2, 3]
operation(vals)
print(vals)  # What is vals here?

If operation is typed with Sequence we can usually assume that vals isn't mutated by it. However, right now it wouldn't be a type error for operation to be implemented like this, which breaks that indication.

def operation(vals: Sequence[int]) -> None:
    assert isinstance(vals, list)
    vals += [3, 4, 5]

Here's the above example on mypy playground.

Pitch

I think it would be a valuable feature for mypy to warn when it narrows from a known immutable type such as Sequence to a known mutable type such as list.

Comienza con el ejemplo enlazado de mypy playground y revisa cómo se gestiona actualmente el narrowing de Sequence a list. Se considera completado cuando mypy pueda identificar este narrowing de un tipo inmutable conocido a un tipo mutable conocido y emitir la advertencia propuesta sin cambiar el comportamiento de otros narrowing.