fix str.join with and/or expression narrowing #2875

[asukaminato0721]

Summary

Fixes #2875

boolean-expression inference now keeps the truthiness-refined slice of a non-final operand instead of the full operand type, which stops (e and e.__name__) or "None" from leaking type[Any] into the result.

Test Plan

add test

[Copilot]

Pull request overview

Fixes a Pyrefly-specific narrowing issue where and/or boolean expressions involving type | None could leave an imprecise type that then broke overload resolution for str.join (issue #2875).

Changes:

• Generalize boolop result-type narrowing by reusing atomic_narrow(..., IsTruthy/IsFalsy) for intermediate operands.
• Make Type::Type(_) statically truthy in Type::as_bool() to support correct and/or narrowing for class objects.
• Add a regression test ensuring sorted((e and e.__name__) or "None" ...) produces list[str] and is accepted by ", ".join(...).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
pyrefly/lib/test/literal.rs	Adds regression test covering str.join with and/or narrowing over `type
pyrefly/lib/alt/narrow.rs	Exposes atomic_narrow to other alt modules (pub(crate)) so boolop inference can reuse it.
pyrefly/lib/alt/expr.rs	Updates boolop inference to narrow intermediate results via atomic_narrow(IsTruthy/IsFalsy) instead of bespoke literal cases.
crates/pyrefly_types/src/types.rs	Treats Type::Type(_) as statically truthy for as_bool(), enabling the intended narrowing behavior.

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[github-actions]

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

steam.py (https://github.com/Gobot1234/steam.py)
- ERROR steam/ext/commands/commands.py:826:11-25: `type[Command] | type[C]` is not assignable to variable `cls` with type `type[C]` [bad-assignment]

[github-actions]

Primer Diff Classification

✅ 1 improvement(s) | 1 project(s) total | -1 errors

1 improvement(s) across steam.py.

Project	Verdict	Changes	Error Kinds	Root Cause
steam.py	✅ Improvement	-1	bad-assignment	crates/pyrefly_types/src/types.rs

Detailed analysis

✅ Improvement (1)

steam.py (-1)

This is an improvement. The PR correctly identifies that type[X] objects are always truthy in Python (no class object is ever falsy). This allows pyrefly to properly narrow cls or Command where cls: type[C] to just type[C], eliminating the false positive bad-assignment error. The old error was wrong because it included type[Command] in the result type of an or expression whose left operand (type[C]) is always truthy — meaning the right operand (Command) is unreachable.
Attribution: The change in crates/pyrefly_types/src/types.rs adding Type::Type(_) => Some(true) is the direct cause. This tells pyrefly that type[X] values are always truthy. Combined with the refactored boolean expression narrowing in pyrefly/lib/alt/expr.rs (which now uses atomic_narrow with IsTruthy/IsFalsy instead of ad-hoc special cases), pyrefly correctly determines that in cls or Command where cls: type[C], the left operand is always truthy, so the result type is just type[C] (not type[C] | type[Command]).

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_{Classification by primer-classifier (1 LLM)}