Type-narrowing Union with Literal using comparison to Literal variable

[Garrett-R]

Bug Report

Strangely, type-narrowing for unions including a Literal works for int but not datetime

To Reproduce

Mypy Playground. Copied here for convenience:

from datetime import datetime
from typing import Final, Literal

Unspecified = Literal['unspecified']
UNSPECIFIED: Final[Unspecified] = 'unspecified'

def do_thing(x: datetime | Unspecified) -> None:
 if x != UNSPECIFIED:
 print(x.utcnow()) # Unexpected error!

def do_thing_2(x: int | Unspecified) -> None:
 if x != UNSPECIFIED:
 print(x.real) # No prob

Expected Behavior

No errors.

Actual Behavior

Has errors:

main.py:9: error: Item "str" of "datetime | Literal['unspecified']" has no attribute "utcnow" [union-attr]

Your Environment

• Mypy version used: 1.7.0
• Mypy command-line flags: <none>
• Mypy configuration options from mypy.ini (and other config files): <none>
• Python version used: 3.11

[vnmabus]

I would say that this is indeed a bug. It won't be a bug in the positive case (as a class that redefines __eq__ MAY be equal to a string), but it should definitely narrow in the negative case. Here you can see that the problem manifest itself depending on if you redefined __eq__ or not:

from datetime import datetime
from typing import Final, Literal

Unspecified = Literal['unspecified']
UNSPECIFIED: Final[Unspecified] = 'unspecified'

class KK():
 
 def kk(self):
 pass

class KK2():
 
 def kk(self):
 pass
 
 def __eq__(self, other):
 return False

def do_thing(x: datetime | Unspecified) -> None:
 if x != UNSPECIFIED:
 print(x.utcnow()) # Unexpected error!

def do_thing_2(x: int | Unspecified) -> None:
 if x != UNSPECIFIED:
 print(x.real) # No prob
 
def do_thing_3(x: KK | Unspecified) -> None:
 if x != UNSPECIFIED:
 print(x.kk) # No prob
 
def do_thing_4(x: KK2 | Unspecified) -> None:
 if x != UNSPECIFIED:
 print(x.kk) # Unexpected error!

[A5rocks]

Copying over my smaller reproducer from #18569:

from typing import Literal

class A:
 def __eq__(self, other: object) -> bool: # necessary
 return isinstance(other, A)

def f(v: A | Literal["text"]) -> A | None:
 if v == "text":
 reveal_type(v) # N: Revealed type is "Union[__main__.A, Literal['text']]"
 return None
 else:
 reveal_type(v) # N: Revealed type is "Union[__main__.A, Literal['text']]"
 return v