[1.20 regression] Any is incorrectly narrowed after `is` comparison

[WouldYouKindly]

Hi, this started happening after I upgraded to 1.20 today.

xxx: Any = object()

def func() -> None:
 if 1 is not xxx:
 return
 reveal_type(xxx) # Revealed type is "Literal[1]"

This lead to the code that utilises sentinel objects breaking:

_missing: Any = object()

def func(
 alias: str | None = _missing,
 id_: UUID | None = _missing,
) -> bool:
 return alias is not _missing or id_ is not _missing # Non-overlapping identity check (left operand type: "UUID | None", right operand type: "str | None") [comparison-overlap]

Your Environment

• Mypy version used: mypy 1.20.0 (compiled: yes)
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): strict
• Python version used: 3.13.5

[ilevkivskyi]

cc @hauntsaninja

[ilevkivskyi]

FWIW this looks pretty bad as this is a common pattern.

[hauntsaninja]

In general, mypy will narrow away from Any, so that part is expected. That said, this is a reasonable pattern to support. Current solution I'm thinking of is #21142 , let me know if you don't like it