fix Narrowing for isinstance and type[T] is unsound #713 by asukaminato0721 · Pull Request #2284 · facebook/pyrefly

asukaminato0721 · 2026-02-01T13:36:40Z

Summary

Fixes #713

Adjusted isinstance negative narrowing so we only subtract when the classinfo is a concrete class object (a class def or alias to one), avoiding the unsound narrowing for type[T].

Adjusted negative isinstance narrowing so we only treat type[...] classinfo as concrete when the classinfo expression is a precise literal (class defs combined with | or tuple literals).

Test Plan

Added a regression test that mirrors the issue case to ensure the else branch stays at int | str.

Copilot

Pull request overview

Fixes unsound negative narrowing for isinstance(x, cls) when cls has type type[T] by only performing negative subtraction when the classinfo is a concrete class object.

Changes:

Restrict negative isinstance narrowing to only subtract when classinfo is a concrete ClassDef (or an alias to one).
Add helper methods to detect concrete classinfo for negative narrowing.
Add a regression test ensuring the else branch does not narrow for cls: type[int].

Reviewed changes

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

File	Description
pyrefly/lib/alt/narrow.rs	Adjusts negative `isinstance` narrowing to avoid subtracting for non-concrete classinfo like `type[T]`.
pyrefly/lib/test/narrow.rs	Adds regression coverage to ensure `type[T]` does not trigger unsound negative narrowing.

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github-actions · 2026-02-01T14:22:52Z

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

pytest-robotframework (https://github.com/detachhead/pytest-robotframework)
- ERROR pytest_robotframework/__init__.py:136:35-37: `TracebackType | None` is not assignable to attribute `__traceback__` with type `Never` [bad-assignment]
- ::error file=pytest_robotframework/__init__.py,line=136,col=35,endLine=136,endColumn=37,title=Pyrefly bad-assignment::`TracebackType | None` is not assignable to attribute `__traceback__` with type `Never`

kopf (https://github.com/nolar/kopf)
- ERROR kopf/_cogs/structs/dicts.py:259:14-38: Runtime checkable protocol `Iterable` has an unsafe overlap with type `Never` [unsafe-overlap]
- ::error file=kopf/_cogs/structs/dicts.py,line=259,col=14,endLine=259,endColumn=38,title=Pyrefly unsafe-overlap::Runtime checkable protocol `Iterable` has an unsafe overlap with type `Never`%0A  Attribute `__iter__` has incompatible types: `Never.__iter__` has type `Never`, which is not consistent with `BoundMethod[Never, (self: Never) -> Iterator[@_]]` in `Iterable.__iter__` (the type of read-write attributes cannot be changed)

porcupine (https://github.com/Akuli/porcupine)
- ERROR porcupine/plugins/highlight/pygments_highlighter.py:51:12-52: Object of class `type` has no attribute `get_tokens_unprocessed` [missing-attribute]
- ::error file=porcupine/plugins/highlight/pygments_highlighter.py,line=51,col=12,endLine=51,endColumn=52,title=Pyrefly missing-attribute::Object of class `type` has no attribute `get_tokens_unprocessed`

ibis (https://github.com/ibis-project/ibis)
- ::error file=ibis/expr/operations/window.py,line=50,col=9,endLine=50,endColumn=19,title=Pyrefly bad-override::Class member `WindowBoundary.__coerce__` overrides parent class `Value` in an inconsistent manner%0A  `WindowBoundary.__coerce__` has type `BoundMethod[type[WindowBoundary[T, S]], (cls: type[WindowBoundary[T, S]], value: Unknown, **kwargs: Unknown) -> WindowBoundary[T, S] | WindowBoundary[Unknown, Any]]`, which is not assignable to `BoundMethod[type[WindowBoundary[T, S]], (cls: type[WindowBoundary[T, S]], value: Any, T: type[Any] | None = None, S: type[Any] | None = None) -> WindowBoundary[T, S]]`, the type of `Value.__coerce__`
+ ::error file=ibis/expr/operations/window.py,line=50,col=9,endLine=50,endColumn=19,title=Pyrefly bad-override::Class member `WindowBoundary.__coerce__` overrides parent class `Value` in an inconsistent manner%0A  `WindowBoundary.__coerce__` has type `BoundMethod[type[WindowBoundary[T, S]], (cls: type[WindowBoundary[T, S]], value: Unknown, **kwargs: Unknown) -> WindowBoundary[Interval | Numeric, Any] | WindowBoundary[T, S] | WindowBoundary[Unknown, Scalar] | WindowBoundary[Unknown, Any]]`, which is not assignable to `BoundMethod[type[WindowBoundary[T, S]], (cls: type[WindowBoundary[T, S]], value: Any, T: type[Any] | None = None, S: type[Any] | None = None) -> WindowBoundary[T, S]]`, the type of `Value.__coerce__`

xarray (https://github.com/pydata/xarray)
+ ERROR xarray/core/dataarray.py:1491:51-57: Argument `Mapping[Any, T_ChunkDimFreq] | tuple[int, ...] | None` is not assignable to parameter `pos_kwargs` with type `Mapping[Any, T_ChunkDimFreq] | None` in function `xarray.namedarray.utils.either_dict_or_kwargs` [bad-argument-type]
+ ERROR xarray/namedarray/core.py:818:44-50: Argument `Mapping[Any, T_ChunkDim] | dict[Any, T_ChunkDim] | tuple[int, ...]` is not assignable to parameter `pos_kwargs` with type `Mapping[Any, T_ChunkDim] | None` in function `xarray.namedarray.utils.either_dict_or_kwargs` [bad-argument-type]
+ ::error file=xarray/core/dataarray.py,line=1491,col=51,endLine=1491,endColumn=57,title=Pyrefly bad-argument-type::Argument `Mapping[Any, T_ChunkDimFreq] | tuple[int, ...] | None` is not assignable to parameter `pos_kwargs` with type `Mapping[Any, T_ChunkDimFreq] | None` in function `xarray.namedarray.utils.either_dict_or_kwargs`
+ ::error file=xarray/namedarray/core.py,line=818,col=44,endLine=818,endColumn=50,title=Pyrefly bad-argument-type::Argument `Mapping[Any, T_ChunkDim] | dict[Any, T_ChunkDim] | tuple[int, ...]` is not assignable to parameter `pos_kwargs` with type `Mapping[Any, T_ChunkDim] | None` in function `xarray.namedarray.utils.either_dict_or_kwargs`

strawberry (https://github.com/strawberry-graphql/strawberry)
+ ERROR strawberry/http/__init__.py:28:26-39: Object of class `NoneType` has no attribute `errors` [missing-attribute]
+ ERROR strawberry/http/__init__.py:28:41-58: Object of class `NoneType` has no attribute `extensions` [missing-attribute]
+ ERROR strawberry/http/__init__.py:30:17-28: Object of class `NoneType` has no attribute `data` [missing-attribute]
+ ERROR strawberry/schema/schema.py:541:12-25: Object of class `NoneType` has no attribute `errors` [missing-attribute]
+ ERROR strawberry/schema/schema.py:542:44-57: `list[GraphQLError] | object | Unknown` is not assignable to attribute `pre_execution_errors` with type `list[GraphQLError] | None` [bad-assignment]
+ ERROR strawberry/schema/schema.py:544:38-51: Argument `list[GraphQLError] | object | Unknown` is not assignable to parameter `errors` with type `list[GraphQLError]` in function `strawberry.schema.base.BaseSchema._process_errors` [bad-argument-type]
+ ERROR strawberry/schema/schema.py:546:63-76: Argument `list[GraphQLError] | object | Unknown | None` is not assignable to parameter `errors` with type `list[GraphQLError] | None` in function `strawberry.types.execution.ExecutionResult.__init__` [bad-argument-type]
+ ERROR strawberry/schema/schema.py:547:9-26: Object of class `NoneType` has no attribute `extensions` [missing-attribute]
+ ERROR strawberry/schema/schema.py:549:16-22: Returned type `ExecutionResult | NoneType | Unknown` is not assignable to declared return type `ExecutionResult` [bad-return]
+ ::error file=strawberry/http/__init__.py,line=28,col=26,endLine=28,endColumn=39,title=Pyrefly missing-attribute::Object of class `NoneType` has no attribute `errors`
+ ::error file=strawberry/http/__init__.py,line=28,col=41,endLine=28,endColumn=58,title=Pyrefly missing-attribute::Object of class `NoneType` has no attribute `extensions`
+ ::error file=strawberry/http/__init__.py,line=30,col=17,endLine=30,endColumn=28,title=Pyrefly missing-attribute::Object of class `NoneType` has no attribute `data`
+ ::error file=strawberry/schema/schema.py,line=541,col=12,endLine=541,endColumn=25,title=Pyrefly missing-attribute::Object of class `NoneType` has no attribute `errors`
+ ::error file=strawberry/schema/schema.py,line=542,col=44,endLine=542,endColumn=57,title=Pyrefly bad-assignment::`list[GraphQLError] | object | Unknown` is not assignable to attribute `pre_execution_errors` with type `list[GraphQLError] | None`
+ ::error file=strawberry/schema/schema.py,line=544,col=38,endLine=544,endColumn=51,title=Pyrefly bad-argument-type::Argument `list[GraphQLError] | object | Unknown` is not assignable to parameter `errors` with type `list[GraphQLError]` in function `strawberry.schema.base.BaseSchema._process_errors`
+ ::error file=strawberry/schema/schema.py,line=546,col=63,endLine=546,endColumn=76,title=Pyrefly bad-argument-type::Argument `list[GraphQLError] | object | Unknown | None` is not assignable to parameter `errors` with type `list[GraphQLError] | None` in function `strawberry.types.execution.ExecutionResult.__init__`
+ ::error file=strawberry/schema/schema.py,line=547,col=9,endLine=547,endColumn=26,title=Pyrefly missing-attribute::Object of class `NoneType` has no attribute `extensions`
+ ::error file=strawberry/schema/schema.py,line=549,col=16,endLine=549,endColumn=22,title=Pyrefly bad-return::Returned type `ExecutionResult | NoneType | Unknown` is not assignable to declared return type `ExecutionResult`

pandera (https://github.com/pandera-dev/pandera)
+ ERROR pandera/backends/ibis/base.py:168:17-36: Object of class `NoneType` has no attribute `rename` [missing-attribute]
+ ERROR pandera/backends/polars/base.py:57:34-50: Object of class `object` has no attribute `sample` [missing-attribute]
+ ERROR pandera/backends/polars/base.py:178:24-49: Object of class `NoneType` has no attribute `columns` [missing-attribute]
+ ERROR pandera/backends/polars/base.py:181:40-70: Object of class `NoneType` has no attribute `with_columns` [missing-attribute]
+ ERROR pandera/backends/polars/base.py:183:29-51: Object of class `NoneType` has no attribute `rows` [missing-attribute]
+ ERROR pandera/backends/polars/base.py:187:40-64: Object of class `NoneType` has no attribute `rename` [missing-attribute]
+ ERROR pandera/backends/polars/base.py:188:26-54: Cannot index into `object` [bad-index]
+ ::error file=pandera/backends/ibis/base.py,line=168,col=17,endLine=168,endColumn=36,title=Pyrefly missing-attribute::Object of class `NoneType` has no attribute `rename`
+ ::error file=pandera/backends/polars/base.py,line=57,col=34,endLine=57,endColumn=50,title=Pyrefly missing-attribute::Object of class `object` has no attribute `sample`
+ ::error file=pandera/backends/polars/base.py,line=178,col=24,endLine=178,endColumn=49,title=Pyrefly missing-attribute::Object of class `NoneType` has no attribute `columns`
+ ::error file=pandera/backends/polars/base.py,line=181,col=40,endLine=181,endColumn=70,title=Pyrefly missing-attribute::Object of class `NoneType` has no attribute `with_columns`
+ ::error file=pandera/backends/polars/base.py,line=183,col=29,endLine=183,endColumn=51,title=Pyrefly missing-attribute::Object of class `NoneType` has no attribute `rows`
+ ::error file=pandera/backends/polars/base.py,line=187,col=40,endLine=187,endColumn=64,title=Pyrefly missing-attribute::Object of class `NoneType` has no attribute `rename`
+ ::error file=pandera/backends/polars/base.py,line=188,col=26,endLine=188,endColumn=54,title=Pyrefly bad-index::Cannot index into `object`%0A  Object of class `object` has no attribute `__getitem__`

websockets (https://github.com/aaugustin/websockets)
+ ERROR src/websockets/sync/connection.py:497:34-42: Runtime checkable protocol `Iterable` has an unsafe overlap with type `Iterable[DataLike] | bytearray | bytes | memoryview[int]` [unsafe-overlap]
+ ::error file=src/websockets/sync/connection.py,line=497,col=34,endLine=497,endColumn=42,title=Pyrefly unsafe-overlap::Runtime checkable protocol `Iterable` has an unsafe overlap with type `Iterable[DataLike] | bytearray | bytes | memoryview[int]`%0A  Attribute `__iter__` has incompatible types: `Iterable[DataLike] | bytearray | bytes | memoryview[int].__iter__` has type `BoundMethod[bytearray, (self: bytearray) -> Iterator[int]]`, which is not assignable to `BoundMethod[Iterable[DataLike] | bytearray | bytes | memoryview[int], (self: Iterable[DataLike] | bytearray | bytes | memoryview[int]) -> Iterator[@_]]`, the type of `Iterable.__iter__`

pylint (https://github.com/pycqa/pylint)
+ ERROR pylint/checkers/async_checker.py:74:25-41: Object of class `NoneType` has no attribute `getattr` [missing-attribute]
+ ERROR pylint/checkers/async_checker.py:75:25-41: Object of class `NoneType` has no attribute `getattr` [missing-attribute]
+ ERROR pylint/checkers/typecheck.py:1940:25-41: Object of class `NoneType` has no attribute `getattr` [missing-attribute]
+ ERROR pylint/checkers/typecheck.py:1941:25-41: Object of class `NoneType` has no attribute `getattr` [missing-attribute]
+ ERROR pylint/checkers/typecheck.py:1957:69-82: Object of class `NoneType` has no attribute `name` [missing-attribute]
+ ::error file=pylint/checkers/async_checker.py,line=74,col=25,endLine=74,endColumn=41,title=Pyrefly missing-attribute::Object of class `NoneType` has no attribute `getattr`
+ ::error file=pylint/checkers/async_checker.py,line=75,col=25,endLine=75,endColumn=41,title=Pyrefly missing-attribute::Object of class `NoneType` has no attribute `getattr`
+ ::error file=pylint/checkers/typecheck.py,line=1940,col=25,endLine=1940,endColumn=41,title=Pyrefly missing-attribute::Object of class `NoneType` has no attribute `getattr`
+ ::error file=pylint/checkers/typecheck.py,line=1941,col=25,endLine=1941,endColumn=41,title=Pyrefly missing-attribute::Object of class `NoneType` has no attribute `getattr`
+ ::error file=pylint/checkers/typecheck.py,line=1957,col=69,endLine=1957,endColumn=82,title=Pyrefly missing-attribute::Object of class `NoneType` has no attribute `name`

stone (https://github.com/dropbox/stone)
- ERROR stone/backends/python_rsrc/stone_base.py:105:75-90: Expected class object, got `Never` [invalid-argument]
- ERROR stone/backends/python_rsrc/stone_base.py:155:69-83: Expected class object, got `Never` [invalid-argument]
- ::error file=stone/backends/python_rsrc/stone_base.py,line=105,col=75,endLine=105,endColumn=90,title=Pyrefly invalid-argument::Expected class object, got `Never`
- ::error file=stone/backends/python_rsrc/stone_base.py,line=155,col=69,endLine=155,endColumn=83,title=Pyrefly invalid-argument::Expected class object, got `Never`

core (https://github.com/home-assistant/core)
+ ERROR homeassistant/components/deconz/entity.py:141:39-61: Object of class `object` has no attribute `reachable` [missing-attribute]
+ ERROR homeassistant/components/litterrobot/entity.py:57:61-69: Object of class `object` has no attribute `id` [missing-attribute]
+ ERROR homeassistant/components/shelly/coordinator.py:246:62-82: Object of class `object` has no attribute `settings` [missing-attribute]
- ERROR homeassistant/components/ssdp/scanner.py:538:23-37: Cannot set item in `dict[Never, Never]` [unsupported-operation]
- ERROR homeassistant/components/ssdp/scanner.py:538:41-44: Cannot set item in `dict[Never, Never]` [unsupported-operation]
- ERROR homeassistant/components/ssdp/scanner.py:549:14-23: Argument `dict[Unknown, Unknown] | dict[Never, Never]` is not assignable to parameter `upnp` with type `Mapping[str, Any]` in function `homeassistant.helpers.service_info.ssdp.SsdpServiceInfo.__init__` [bad-argument-type]
+ ::error file=homeassistant/components/deconz/entity.py,line=141,col=39,endLine=141,endColumn=61,title=Pyrefly missing-attribute::Object of class `object` has no attribute `reachable`
+ ::error file=homeassistant/components/litterrobot/entity.py,line=57,col=61,endLine=57,endColumn=69,title=Pyrefly missing-attribute::Object of class `object` has no attribute `id`
+ ::error file=homeassistant/components/shelly/coordinator.py,line=246,col=62,endLine=246,endColumn=82,title=Pyrefly missing-attribute::Object of class `object` has no attribute `settings`
- ::error file=homeassistant/components/ssdp/scanner.py,line=538,col=23,endLine=538,endColumn=37,title=Pyrefly unsupported-operation::Cannot set item in `dict[Never, Never]`%0A  Argument `Literal['UDN']` is not assignable to parameter `key` with type `Never` in function `dict.__setitem__`
- ::error file=homeassistant/components/ssdp/scanner.py,line=538,col=41,endLine=538,endColumn=44,title=Pyrefly unsupported-operation::Cannot set item in `dict[Never, Never]`%0A  Argument `str` is not assignable to parameter `value` with type `Never` in function `dict.__setitem__`
- ::error file=homeassistant/components/ssdp/scanner.py,line=549,col=14,endLine=549,endColumn=23,title=Pyrefly bad-argument-type::Argument `dict[Unknown, Unknown] | dict[Never, Never]` is not assignable to parameter `upnp` with type `Mapping[str, Any]` in function `homeassistant.helpers.service_info.ssdp.SsdpServiceInfo.__init__`

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Copilot · 2026-02-01T14:38:32Z

+                let allow_unwrap = matches!(right, Type::ClassDef(_))
+                    || (allow_type_form_classinfo
+                        && matches!(&right, Type::Type(box Type::ClassType(_))));
+                if allow_unwrap
+                    && let Some((tparams, right)) = self.unwrap_class_object_silently(&right)
+                {


matches!(right, Type::ClassDef(_)) pattern-matches by value and will move right, but right is then borrowed again in unwrap_class_object_silently(&right), which will not compile. Use a borrowed match (e.g., match on &right) so right isn't moved when computing allow_unwrap.

rchen152 · 2026-02-05T18:54:39Z

I'll take this one, since I've looked at a couple similar narrowing PRs recently.

meta-codesync · 2026-02-06T03:16:50Z

@rchen152 has imported this pull request. If you are a Meta employee, you can view this in D92474106.

stroxler

Review automatically exported from Phabricator review in Meta.

meta-codesync · 2026-02-06T23:12:51Z

@rchen152 merged this pull request in abe13f0.

fix

1f68213

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fix

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fix Narrowing for isinstance and type[T] is unsound #713#2284

fix Narrowing for isinstance and type[T] is unsound #713#2284
asukaminato0721 wants to merge 2 commits intofacebook:mainfrom
asukaminato0721:713

asukaminato0721 commented Feb 1, 2026 •

edited

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asukaminato0721 commented Feb 1, 2026 •

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