sorted() loses information about namedtuple elements

Bug Report

it seems that running sorted() on an union of iterables of named tuple, create some missing information

To Reproduce

playground url

from typing import NamedTuple
from typing import reveal_type


class NamedTupleA(NamedTuple):
    field_a: str


class NamedTupleB(NamedTuple):
    field_b: str


def my_fun(my_list: list[NamedTupleA] | list[NamedTupleB]) -> None:
    reveal_type(my_list)
    # Revealed type is "Union[builtins.list[tuple[builtins.str, fallback=__main__.NamedTupleA]], builtins.list[tuple[builtins.str, fallback=__main__.NamedTupleB]]]"

    for nt in my_list:
        if isinstance(nt, NamedTupleA):
            print(nt.field_a)
        else:
            # here nt is recognized of `NamedTupleB` type, I can access field_b
            print(nt.field_b)

    reveal_type(sorted(my_list))
    # Revealed type is "builtins.list[tuple[builtins.str]]"

    for sorted_nt in sorted(my_list):
        if isinstance(sorted_nt, NamedTupleA):
            print(sorted_nt.field_a)
        else:
            # here I expected nt to be recognized of `NamedTupleB` type
            print(sorted_nt.field_b)

if __name__ == '__main__':
    my_fun([NamedTupleA('value_1')])
    my_fun([NamedTupleB('value_2')])
    # this error is expected: I cannot mix NamedTupleA and NamedTupleB
    my_fun([NamedTupleA('value_1'), NamedTupleB('value_2')])

basically I have 2 list of NamedTuples.
my_fun must accept either one.
Ideally I could write a User-Defined Type Guards, but let's say that I'm just checking (with isinstance) each element type.

If I loop directly over the list, the check works for both the if and the else branch (there shouldn't be any other case, the union is between two types)

If I loop directly over the sorted list, the type of the element is different, so in the else branch mypy gives me an error

Expected Behavior

I expect no errors on print(sorted_nt.field_b)

Actual Behavior

main.py:32: error: "tuple[str]" has no attribute "field_b" [attr-defined]

Your Environment

linux, python3.12, mypy 1.10.1

Inizia con la riproduzione collegata su mypy playground usando Python 3.12 e mypy 1.10.1, quindi confronta i tipi rivelati prima e dopo sorted() in main.py. La correzione è completa quando il risultato di sorted() preserva le alternative di NamedTuple, in modo che i rami isinstance accettino sorted_nt.field_a e sorted_nt.field_b senza un errore attr-defined.