Fix negative type narrowing for isinstance/issubclass with type[T] variable or tuple

[Henry Su (hsusul)]

Summary

Fixes an issue where isinstance() and issubclass() type narrowing failed to extract class filters when passed a variable of type type[T] or tuple[type[T], ...]. Also enables sound negative type narrowing when T is a @final class.

Problem & Root Cause

1. getIsInstanceClassTypes extracts element class types for isinstance/issubclass. When given a variable of type type[T] or tuple[type[A], type[B]], the elements in tupleTypeArgs are ClassInstances of type (e.g. type[A]), rather than instantiable class types A. Previously, getIsInstanceClassTypes set foundNonClassType = true on type[T] instances, failing to extract the target filter types and resulting in no type narrowing.
2. In negative narrowing (isPositiveTest = false), non-final classes typed as type[T] must preserve includeSubclasses = true because type[T] may hold a runtime subclass of T. However, when T is @final (e.g. @final class or built-in final type), T cannot have runtime subclasses, making negative narrowing sound.

Fix

1. In getIsInstanceClassTypes:
   • Specifically check for ClassInstances of type (ClassType.isBuiltIn(subtype, 'type')) and extract their type arguments (type[T] -> T).
   • Reject type[Any] or type[Unknown] as concrete class filters.
   • Preserve includeSubclasses = true on extracted instantiable class types.
2. In narrowTypeForInstanceOrSubclassInternal:
   • Allow negative elimination when ClassType.isFinal(concreteFilterType) is true, permitting sound negative narrowing for final class type[T] variables.

Verification

• All 160 unit tests in typeEvaluator1.test.ts pass cleanly (160/160).
• git diff --check, npm run check, and npm run typecheck pass with 0 errors.

[Stella Huang (StellaHuang95)]

🔒 Automated review in progress — Stella Huang (@StellaHuang95) is auto-reviewing this PR.

[Stella Huang (StellaHuang95)]

Clearing includeSubclasses makes negative narrowing for type[T] unsound: a runtime strict subclass can cause an A value to enter an else branch where this change removes A.

[Stella Huang (StellaHuang95)]

Clearing includeSubclasses makes negative narrowing for type[T] filters unsound and regresses existing subclass behavior.

[Stella Huang (StellaHuang95)]

Negative narrowing becomes unsound because type[A] may hold a strict subclass of A, and the new conversion also accepts non-type[...] class instances as valid filters.

[Stella Huang (StellaHuang95)]

Info · Optional note

The instantiable typeArg branch assigns the filter directly, while the instance-form branch converts it and thereby preserves includeSubclasses. Normalize the instantiable branch as well so any future representation reaching it cannot bypass the conservative non-final negative-narrowing behavior. [verified]

[Stella Huang (StellaHuang95)]

Warning · Non-blocking recommendation

Please add a non-final negative case. cls: type[A] may hold a subclass of A, so if not isinstance(x, cls) must retain A and reveal A | B; this is the soundness boundary protected by the new final-class guard. [verified]

[Stella Huang (StellaHuang95)]

Warning · Non-blocking recommendation

The sample and PR claim coverage for both isinstance and issubclass, but this file exercises only isinstance. Add an issubclass(..., cls: type[A]) regression case so its narrowing consumer remains covered. [verified]

[Stella Huang (StellaHuang95)]

Warning · Non-blocking recommendation

Add regression coverage for type[Any] and type[Unknown]. The new foundNonClassType path intentionally rejects these as concrete filters, but the behavior is currently untested.

[Stella Huang (StellaHuang95)]

Approved via Review Center.

[Rich Chiodo (rchiodo)]

Approved via Review Center.

[Rich Chiodo (rchiodo)]

Approved via Review Center.