8000 [1.16 makes more common] Inference of Any against optional type infers Never · Issue #8829 · python/mypy · GitHub
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[1.16 makes more common] Inference of Any against optional type infers Never #8829

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rggjan opened this issue May 15, 2020 · 3 comments · Fixed by #19190
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[1.16 makes more common] Inference of Any against optional type infers Never #8829

rggjan opened this issue May 15, 2020 · 3 comments · Fixed by #19190
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bug mypy got something wrong priority-0-high topic-type-narrowing Conditional type narrowing / binder topic-type-variables

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@rggjan
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rggjan commented May 15, 2020

See the following example:

from typing import Any, Optional, TypeVar

T = TypeVar("T")


def assert_not_none(value: Optional[T]) -> T:
    assert value is not None
    return value


def foo1(a: Optional[Any]) -> int:
    return assert_not_none(a)[3]


def foo2(a: Optional[Any]) -> int:
    assert a is not None
    return a[3]

I would expect that foo1 and foo2 are equivalent. However, running mypy on this file, I get:

test.py:12: error: Value of type <nothing> is not indexable

For some reason, assert_not_none doesn't map Optional[Any] to Any, as it is supposed to (and which would accept the indexing), but instead maps it to <nothing> which throws an error. The second version with asserting that it is not None seems to work fine, though.
@msullivan msullivan added bug mypy got something wrong priority-1-normal labels May 20, 2020
@msullivan
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Definitely a bug and a somewhat surprising one too. Probably an issue in constraints.py?

@AlexWaygood AlexWaygood added topic-type-variables topic-type-narrowing Conditional type narrowing / binder labels Mar 27, 2022
@ilevkivskyi
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The root cause is:

a: Any
def assert_not_none(value: Optional[T]) -> T: ...
reveal_type(assert_not_none(a))  # Revealed type is "Never"

(and same with Optional[Any]) because of loose handling of unions in constraints.py. This is pretty bad IMO, so raising priority to high.

@ilevkivskyi ilevkivskyi changed the title Value of type <nothing> is not indexable Inference of Any against optional type infers Never Feb 8, 2025
@ilevkivskyi ilevkivskyi mentioned this issue Feb 8, 2025
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@JukkaL
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JukkaL commented Apr 30, 2025

I encountered this when preparing an internal codebase for the 1.16 release. The issue wasn't directly related to any change in 1.16, though -- some additional Any | None types are inferred after recent changes.

@hauntsaninja hauntsaninja changed the title Inference of Any against optional type infers Never [1.16 makes more common] Inference of Any against optional type infers Never May 28, 2025
This was referenced May 31, 2025
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hauntsaninja pushed a commit that referenced this issue Jun 1, 2025
@ilevkivskyi
Infer constraints eagerly if actual is Any (#19190)
a16521f 
Fixes #8829

This case is more common in 1.16 due to some changes in binder, so it
would be good to fix it. My fix may be a bit naive, but if `mypy_primer`
looks good, I think it should be OK.
@hauntsaninja hauntsaninja mentioned this issue Jun 3, 2025
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Infer constraints eagerly if actual is Any ilevkivskyi/mypy
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@rggjan @msullivan @JukkaL @ilevkivskyi @AlexWaygood
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