Type narrowing after boolean exhaustion

[JulienPalard]

Feature

Given a Union[X, bool], after the exhaustion of "all" boolean possibilities, the type could be narrowed to X, example:

from random import randint
from typing import Union

str_or_bool: Union[str, bool]

if randint(0, 1):
 str_or_bool = "Hello"
else:
 str_or_bool = False

if str_or_bool is True:
 print("OK")
elif str_or_bool is False:
 print("KO")
elif str_or_bool:
 # If we land in this line, the type can't be bool, so the following expression is valid: 
 print(">" + str_or_bool)

I think similar "exhaustion narrowing" could be done with any type having a limited cardinality of values, like TypedDict keys, unions of literals, ... in if/elif/ chains and maybe in the match statement?

Another example with Union of Union of Literals:

from random import randint
from typing import Union, Literal


Literal1 = Union[Literal["a", "b"]]
Literal2 = Union[Literal["c", "d"]]
thing: Union[Literal1, Literal2]


if randint(0, 1):
 thing = "a"
else:
 thing = "b"

if thing == "a":
 print("OK")
elif thing == "b":
 print("KO")
else:
 reveal_type(thing) # Could be narrowed to Literal2

(But strangely in this case reveal_type shows nothing, is this an issue?)

[hauntsaninja]

This will be fixed by #10389