Warn when class instance without __bool__ or __len__ is used as boolean condition

[knQzx]

Summary

closes #3282 closes #868

adds ConditionRedundantReason::InstanceAlwaysTruthy - fires when a class instance is used in a boolean context but the class defines neither __bool__ nor __len__ (so instances are unconditionally truthy)

class NoBool:
    pass

x = NoBool()
if x:   # warning: NoBool defines neither __bool__ nor __len__, instances are always truthy
    pass

skipped to keep noise down

• object itself
• protocols and ABCs (extends_abc) - concrete subclasses can define the dunders at runtime
• custom metaclasses inheriting from ABCMeta via MRO (covers HA's ABCCachedProperties(CachedProperties, ABCMeta))
• classes from bundled stubs (typeshed stdlib + third-party stubs) - many stub-only classes have dynamic runtime behavior not in stubs (datetime, asyncio.Future, Lock, sqlalchemy Session, etc)
• dataclasses
• classes with __bool__ / __len__ / __getattr__ / __getattribute__ / __get__ anywhere in MRO (descriptors, dynamic attribute interception)

remaining warnings on home-assistant primer

23 warnings, three patterns:

• dead defensive checks (~16): device = random.choice(...) returns non-optional T then code does if device:. dead by types, lint working as intended
• subclass narrowing (~2): base declares attr: T | None, subclass redeclares attr: T. after redeclaration if obj.attr: is dead by types
• lying types via # type: ignore[assignment] (~5): platform: EntityPlatform = None # type: ignore[assignment]. attr annotated T but assigned None with the assignment error suppressed. if self.platform: is meaningful at runtime but dead by declared types. proper fix is T | None. not suppressed because doing so needs initializer tracking on ClassField to hide what are arguably real type-annotation bugs

none look like the "incomplete stubs" failure mode the issue author flagged

Test Plan

tests in flow_branching.rs cover plain classes, inheritance, descriptors, __getattr*__, dataclasses, stub-only classes, protocols, ABCs, custom-metaclass-via-MRO

cargo test -p pyrefly clean

[github-actions]

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

core (https://github.com/home-assistant/core)
+ ERROR homeassistant/components/derivative/sensor.py:430:20-29: Instance of `State` used as condition, but `State` defines neither `__bool__` nor `__len__`, so instances are always truthy. It's equivalent to `True` [redundant-condition]
+ ERROR homeassistant/components/gdacs/sensor.py:103:12-25: Instance of `GdacsFeedEntityManager` used as condition, but `GdacsFeedEntityManager` defines neither `__bool__` nor `__len__`, so instances are always truthy. It's equivalent to `True` [redundant-condition]
+ ERROR homeassistant/components/home_connect/coordinator.py:165:28-49: Instance of `HomeConnectApplianceCoordinator` used as condition, but `HomeConnectApplianceCoordinator` defines neither `__bool__` nor `__len__`, so instances are always truthy. It's equivalent to `True` [redundant-condition]
+ ERROR homeassistant/components/knx/storage/config_store.py:254:12-22: Instance of `KNXModule` used as condition, but `KNXModule` defines neither `__bool__` nor `__len__`, so instances are always truthy. It's equivalent to `True` [redundant-condition]
+ ERROR homeassistant/components/media_player/__init__.py:1349:16-31: Instance of `EntityPlatform` used as condition, but `EntityPlatform` defines neither `__bool__` nor `__len__`, so instances are always truthy. It's equivalent to `True` [redundant-condition]
+ ERROR homeassistant/components/netatmo/media_source.py:122:20-25: Instance of `BrowseMediaSource` used as condition, but `BrowseMediaSource` defines neither `__bool__` nor `__len__`, so instances are always truthy. It's equivalent to `True` [redundant-condition]
+ ERROR homeassistant/components/netatmo/media_source.py:130:20-25: Instance of `BrowseMediaSource` used as condition, but `BrowseMediaSource` defines neither `__bool__` nor `__len__`, so instances are always truthy. It's equivalent to `True` [redundant-condition]
+ ERROR homeassistant/components/onvif/device.py:175:12-23: Instance of `EventManager` used as condition, but `EventManager` defines neither `__bool__` nor `__len__`, so instances are always truthy. It's equivalent to `True` [redundant-condition]
+ ERROR homeassistant/components/owntracks/device_tracker.py:156:12-21: Instance of `HomeAssistant` used as condition, but `HomeAssistant` defines neither `__bool__` nor `__len__`, so instances are always truthy. It's equivalent to `True` [redundant-condition]
+ ERROR homeassistant/components/screenlogic/util.py:40:12-36: Instance of `ConfigEntry` used as condition, but `ConfigEntry` defines neither `__bool__` nor `__len__`, so instances are always truthy. It's equivalent to `True` [redundant-condition]
+ ERROR homeassistant/components/shelly/diagnostics.py:35:16-33: Instance of `ShellyBlockCoordinator` used as condition, but `ShellyBlockCoordinator` defines neither `__bool__` nor `__len__`, so instances are always truthy. It's equivalent to `True` [redundant-condition]
+ ERROR homeassistant/components/solarman/sensor.py:273:16-40: Instance of `ConfigEntry` used as condition, but `ConfigEntry` defines neither `__bool__` nor `__len__`, so instances are always truthy. It's equivalent to `True` [redundant-condition]
+ ERROR homeassistant/components/sonos/media_browser.py:324:8-25: Instance of `SonosFavorites` used as condition, but `SonosFavorites` defines neither `__bool__` nor `__len__`, so instances are always truthy. It's equivalent to `True` [redundant-condition]
+ ERROR homeassistant/components/sql/config_flow.py:121:8-12: Instance of `Session` used as condition, but `Session` defines neither `__bool__` nor `__len__`, so instances are always truthy. It's equivalent to `True` [redundant-condition]
+ ERROR homeassistant/components/sql/config_flow.py:139:12-16: Instance of `Session` used as condition, but `Session` defines neither `__bool__` nor `__len__`, so instances are always truthy. It's equivalent to `True` [redundant-condition]
+ ERROR homeassistant/components/sql/config_flow.py:147:16-20: Instance of `Session` used as condition, but `Session` defines neither `__bool__` nor `__len__`, so instances are always truthy. It's equivalent to `True` [redundant-condition]
+ ERROR homeassistant/components/sql/config_flow.py:155:8-12: Instance of `Session` used as condition, but `Session` defines neither `__bool__` nor `__len__`, so instances are always truthy. It's equivalent to `True` [redundant-condition]
+ ERROR homeassistant/components/sql/util.py:51:16-26: Instance of `HomeAssistant` used as condition, but `HomeAssistant` defines neither `__bool__` nor `__len__`, so instances are always truthy. It's equivalent to `True` [redundant-condition]
+ ERROR homeassistant/components/stream/__init__.py:582:12-15: Instance of `HlsStreamOutput` used as condition, but `HlsStreamOutput` defines neither `__bool__` nor `__len__`, so instances are always truthy. It's equivalent to `True` [redundant-condition]
+ ERROR homeassistant/components/tts/__init__.py:1343:12-27: Instance of `EntityPlatform` used as condition, but `EntityPlatform` defines neither `__bool__` nor `__len__`, so instances are always truthy. It's equivalent to `True` [redundant-condition]
+ ERROR homeassistant/components/vegehub/__init__.py:98:12-23: Instance of `VegeHubCoordinator` used as condition, but `VegeHubCoordinator` defines neither `__bool__` nor `__len__`, so instances are always truthy. It's equivalent to `True` [redundant-condition]
+ ERROR homeassistant/helpers/entity.py:1564:12-25: Instance of `EntityPlatform` used as condition, but `EntityPlatform` defines neither `__bool__` nor `__len__`, so instances are always truthy. It's equivalent to `True` [redundant-condition]
+ ERROR homeassistant/helpers/entity.py:1705:25-89: Instance of `PlatformData` used as condition, but `PlatformData` defines neither `__bool__` nor `__len__`, so instances are always truthy. It's equivalent to `True` [redundant-condition]

[github-actions]

Primer Diff Classification

✅ 1 improvement(s) | 1 project(s) total | +23 errors

1 improvement(s) across core.

Project	Verdict	Changes	Error Kinds	Root Cause
core	✅ Improvement	+23	redundant-condition	class_has_bool_or_len()

Detailed analysis

✅ Improvement (1)

core (+23)

These 23 warnings are technically correct — the flagged class instances are indeed always truthy because their classes define neither __bool__ nor __len__. The PR author analyzed all 23 and confirmed they represent dead defensive checks, subclass narrowing, or lying types via # type: ignore[assignment]. The check has thoughtful exclusions for ABCs, protocols, stubs, dataclasses, and dynamic attribute classes. While this is a novel check not present in mypy/pyright, it correctly identifies genuinely redundant conditions. This is a new lint capability, not a false positive generator.
Attribution: The changes in pyrefly/lib/alt/expr.rs (adding ConditionRedundantReason::InstanceAlwaysTruthy and the Type::ClassType match arm in the condition redundancy checker) directly cause these 23 new warnings. The class_has_bool_or_len() method in pyrefly/lib/alt/attr.rs provides the MRO-walking logic, and metaclass_extends_abcmeta() in pyrefly/lib/alt/class/class_metadata.rs ensures ABCMeta-derived metaclasses are properly excluded.

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