Treat bool as equivalent to Literal[True, False] to allow better type inference

[ahmedsoe]

Similar to this #6027, but more basic use case.

from typing import Collection, Union, List


def foo(bar: Union[bool, Collection[int]]) -> Union[int, List[int]]:
    if bar is False:  # or   if not bar:
        return 10

    if bar is True:
        return 20

    # otherwise, bar must be a collection (list, tuple, ...)
    return [i*2 for i in bar]


print(foo(True))
print(foo(False))
print(foo([1, 2, 3]))
print(foo((1, 2, 3,)))

Mypy is giving the error

error: Item "bool" of "Union[bool, Collection[int]]" has no attribute "__iter__" (not iterable)

But it should be able to infer that it can't be a bool?
using if / elif /else also gives the error.

[ilevkivskyi]

I think this can be done using literal types: essentially we can treat bool as Union[Literal[True], Literal[False]].

cc @Michael0x2a

[ilevkivskyi]

This appeared again so raising priority to normal.

[ethan-leba]

Would like to take a stab at this if no one's picked it up already!