Error feedback for unions of lists that should be unions of sequences is not as helpful as it could be

[toolness]

Sorry this issue has such an odd title, but I was about to report a mypy bug based on the following code:

from typing import List, Union
from enum import Enum


class Boop(Enum):
    FOO = 1
    BAR = 2


def do_thing_with_enums(enums: Union[List[Enum], Enum]) -> None:
    if isinstance(enums, Enum):
        enums = list([enums])
    for thing in enums:
        thing.value


class Funky:
    boop: List[Boop]

    def boop_it_up(self) -> None:
        do_thing_with_enums(self.boop)

The mypy error output for the following code is:

boop.py:21: error: Argument 1 to "do_thing_with_enums" has incompatible type "List[Boop]"; expected "Union[List[Enum], Enum]"

I found this quite confusing since List[Boop] is also a List[Enum], which I thought was a mypy bug.

However, after a lot of tinkering, I simplified the do_thing_with_enums signature to be:

def do_thing_with_enums(enums: List[Enum]) -> None:

at this point mypy still failed, but it gave me much better error feedback:

boop.py:21: error: Argument 1 to "do_thing_with_enums" has incompatible type "List[Boop]"; expected "List[Enum]"
boop.py:21: note: "List" is invariant -- see http://mypy.readthedocs.io/en/latest/common_issues.html#variance
boop.py:21: note: Consider using "Sequence" instead, which is covariant

After converting the type annotation to use Sequence instead of List, everything worked fine. I just wished it had given me such excellent advice for my original type annotation, rather than requiring me to simplify it, if that makes sense.

[remilapeyre]

I made a very naive PR to display a note when this happen. I will be happy to improve it when you have time to review :)