[1.20 regression] Inconsistent narroving for unions involving Any

@hauntsaninja

Consider this example:

from typing import Any

class B:
    x: int

class C(B):
    y: int

x: B | int
y: C | Any

assert x == y
reveal_type(x)  # Revealed type is "test.B | int", OK
reveal_type(y)  # Revealed type is "Any | test.B | int", why???
y.y  # Error here, previously this worked.

cc @hauntsaninja I didn't bisect, but there is high chance this may be caused by one of the recent narrowing changes.

Yeah, this will be related to my change for custom equality

I guess there are two issues here: 1) inconsistency between union case and non union case, 2) violation of gradual typing principle

For the inconsistency, these should both either be Any or Any | B | int

from typing import Any

class B: ...

def f1(x: B | int, y: Any):
    if x == y:
        reveal_type(x)  # B | int
        reveal_type(y)  # Any

def f2(x: B | int, y: Any | None):
    if x == y:
        reveal_type(x)  # B | int
        reveal_type(y)  # Any | B | int

In general I really do like how willing mypy is to turn Any into stricter types, but I think in this case preserving gradual guarantee and just sticking with Any will work better (certainly on your example). I will still check primer for the Any | B | int option to see if I learn something interesting...

Okay I see, this will fix the issue mentioned in this comment #21199 (comment)

Okay yes, primer for the non-union change confirms preserving gradual guarantee here is important in practice as well

Preserve gradual guarantee when narrowing Any union via equality (#21368

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[1.20 regression] Inconsistent narroving for unions involving Any #21364

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[1.20 regression] Inconsistent narroving for unions involving Any #21364

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hauntsaninja commented on Apr 29, 2026

hauntsaninja commented on Apr 29, 2026

hauntsaninja commented on Apr 29, 2026

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