fix using += on types without __iadd__ will evaluate __getattr__ instead of __add__ #2248

[asukaminato0721]

Summary

Fixes #2248

Fix scoped operator lookup to ignore __getattr__ and added a regression test for += when __iadd__ is missing. This aligns augmented assignment with Python’s runtime semantics for magic methods and prevents __getattr__ from hijacking __iadd__/__add__ resolution.

Test Plan

add a test

[Copilot]

Pull request overview

This PR fixes a bug where augmented assignment operators (like +=) incorrectly consulted __getattr__ when the in-place dunder method (like __iadd__) was not found, instead of falling back to the regular operator method (like __add__). This aligns Pyrefly with Python's runtime semantics and the behavior of other type checkers (mypy, pyright, basedpyright, ty).

Changes:

• Changed operator magic method lookup to ignore __getattr__/__getattribute__ by setting allow_getattr_fallback=false
• Added a regression test verifying that += correctly falls back to __add__ when __iadd__ is missing, even when __getattr__ is present

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated no comments.

File	Description
pyrefly/lib/test/operators.rs	Added test case test_iadd_ignores_getattr that verifies augmented assignment ignores __getattr__ and correctly falls back to the regular operator method
pyrefly/lib/alt/operators.rs	Fixed try_binop_calls to pass false for allow_getattr_fallback parameter, ensuring operator lookups bypass __getattr__
Cargo.lock	Automatic checksum update for backtrace dependency (unrelated to the fix)

---

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[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

openlibrary (https://github.com/internetarchive/openlibrary)
+ ERROR openlibrary/plugins/openlibrary/lists.py:811:43-813:18: Cannot set item in `dict[str, list[dict[Unknown, Unknown]]]` [unsupported-operation]
+ ERROR openlibrary/plugins/openlibrary/lists.py:818:40-88: Cannot set item in `dict[str, list[dict[Unknown, Unknown]]]` [unsupported-operation]
+ ERROR openlibrary/plugins/openlibrary/lists.py:823:42-825:18: Cannot set item in `dict[str, list[dict[Unknown, Unknown]]]` [unsupported-operation]
+ ERROR openlibrary/plugins/upstream/tests/test_merge_authors.py:152:12-52: Object of class `NoneType` has no attribute `key` [missing-attribute]
+ ::error file=openlibrary/plugins/openlibrary/lists.py,line=811,col=43,endLine=813,endColumn=18,title=Pyrefly unsupported-operation::Cannot set item in `dict[str, list[dict[Unknown, Unknown]]]`%0A  Argument `list[Thing | Unknown]` is not assignable to parameter `value` with type `list[dict[Unknown, Unknown]]` in function `dict.__setitem__`
+ ::error file=openlibrary/plugins/openlibrary/lists.py,line=818,col=40,endLine=818,endColumn=88,title=Pyrefly unsupported-operation::Cannot set item in `dict[str, list[dict[Unknown, Unknown]]]`%0A  Argument `list[Thing | Unknown]` is not assignable to parameter `value` with type `list[dict[Unknown, Unknown]]` in function `dict.__setitem__`
+ ::error file=openlibrary/plugins/openlibrary/lists.py,line=823,col=42,endLine=825,endColumn=18,title=Pyrefly unsupported-operation::Cannot set item in `dict[str, list[dict[Unknown, Unknown]]]`%0A  Argument `list[Thing | Unknown]` is not assignable to parameter `value` with type `list[dict[Unknown, Unknown]]` in function `dict.__setitem__`
- ::error file=openlibrary/plugins/upstream/tests/test_merge_authors.py,line=37,col=9,endLine=37,endColumn=12,title=Pyrefly bad-override::Class member `MockSite.get` overrides parent class `Site` in an inconsistent manner%0A  `MockSite.get` has type `BoundMethod[MockSite, (self: MockSite, key: Unknown) -> Unknown]`, which is not assignable to `BoundMethod[MockSite, (self: MockSite, key: Unknown, revision: Unknown | None = None, lazy: bool | Unknown = False) -> Unknown | None]`, the type of `Site.get`
+ ::error file=openlibrary/plugins/upstream/tests/test_merge_authors.py,line=37,col=9,endLine=37,endColumn=12,title=Pyrefly bad-override::Class member `MockSite.get` overrides parent class `Site` in an inconsistent manner%0A  `MockSite.get` has type `BoundMethod[MockSite, (self: MockSite, key: Unknown) -> Thing | Unknown]`, which is not assignable to `BoundMethod[MockSite, (self: MockSite, key: Unknown, revision: Unknown | None = None, lazy: bool | Unknown = False) -> Thing | Unknown | None]`, the type of `Site.get`
+ ::error file=openlibrary/plugins/upstream/tests/test_merge_authors.py,line=152,col=12,endLine=152,endColumn=52,title=Pyrefly missing-attribute::Object of class `NoneType` has no attribute `key`

streamlit (https://github.com/streamlit/streamlit)
- ERROR lib/streamlit/hello/dataframe_demo.py:40:13-30: `/=` is not supported between `DataFrame` and `float` [unsupported-operation]
- ::error file=lib/streamlit/hello/dataframe_demo.py,line=40,col=13,endLine=40,endColumn=30,title=Pyrefly unsupported-operation::`/=` is not supported between `DataFrame` and `float`%0A  Expected `__itruediv__` to be a callable, got `Series[Any]`

xarray (https://github.com/pydata/xarray)
+ ERROR xarray/coding/times.py:743:22-67: `%` is not supported between `PandasExtensionArray[ExtensionArray]` and `timedelta` [unsupported-operation]
+ ERROR xarray/coding/times.py:743:22-67: `%` is not supported between `PandasExtensionArray[ExtensionArray]` and `timedelta64[int | timedelta | None]` [unsupported-operation]
+ ::error file=xarray/coding/times.py,line=743,col=22,endLine=743,endColumn=67,title=Pyrefly unsupported-operation::`%25` is not supported between `PandasExtensionArray[ExtensionArray]` and `timedelta`%0A  Cannot find `__mod__` or `__rmod__`
+ ::error file=xarray/coding/times.py,line=743,col=22,endLine=743,endColumn=67,title=Pyrefly unsupported-operation::`%25` is not supported between `PandasExtensionArray[ExtensionArray]` and `timedelta64[int | timedelta | None]`%0A  No matching overload found for function `numpy.timedelta64.__rmod__` called with arguments: (PandasExtensionArray[ExtensionArray])%0A  Possible overloads:%0A  (x: timedelta64[None], /) -> timedelta64[None] [closest match]%0A  (x: timedelta64[int | timedelta | None], /) -> timedelta64[None]%0A  (x: timedelta64[int | timedelta], /) -> timedelta64[int | None]%0A  (x: timedelta64[_AnyTD64Item], /) -> timedelta64[_AnyTD64Item | None]%0A  (x: timedelta, /) -> timedelta%0A  (x: timedelta64[int], /) -> timedelta64[int | None]%0A  (x: timedelta64[int | timedelta | None], /) -> timedelta64[int | timedelta | None]

pandera (https://github.com/pandera-dev/pandera)
- ERROR pandera/engines/pandas_engine.py:541:9-44: `&=` is not supported between `Series[bool]` and `Series[bool]` [unsupported-operation]
- ERROR pandera/engines/pandas_engine.py:543:9-36: `&=` is not supported between `Series[bool]` and `Series[bool]` [unsupported-operation]
- ::error file=pandera/engines/pandas_engine.py,line=541,col=9,endLine=541,endColumn=44,title=Pyrefly unsupported-operation::`&=` is not supported between `Series[bool]` and `Series[bool]`%0A  Expected `__iand__` to be a callable, got `bool`
- ::error file=pandera/engines/pandas_engine.py,line=543,col=9,endLine=543,endColumn=36,title=Pyrefly unsupported-operation::`&=` is not supported between `Series[bool]` and `Series[bool]`%0A  Expected `__iand__` to be a callable, got `bool`

pip (https://github.com/pypa/pip)
- ::error file=src/pip/_vendor/pygments/token.py,line=44,col=16,endLine=44,endColumn=46,title=Pyrefly unsupported-operation::`+` is not supported between `Literal['Token']` and `Self@_TokenType`%0A  Expected `__radd__` to be a callable, got `_TokenType | Any`
+ ::error file=src/pip/_vendor/pygments/token.py,line=44,col=16,endLine=44,endColumn=46,title=Pyrefly unsupported-operation::`+` is not supported between `Literal['Token']` and `Self@_TokenType`%0A  No matching overload found for function `str.__add__` called with arguments: (Self@_TokenType)%0A  Possible overloads:%0A  (value: LiteralString, /) -> LiteralString [closest match]%0A  (value: str, /) -> str

[meta-codesync]

@rchen152 has imported this pull request. If you are a Meta employee, you can view this in D92434131.

[rchen152]

Looks good to me, thank you!

[stroxler]

Review automatically exported from Phabricator review in Meta.

[meta-codesync]

@rchen152 merged this pull request in 8a40ad7.