fix(codegen): invalidate Array.isArray folds on reassignment

[proggeramlug]

Summary

• stop Array.isArray from folding a reassigned local from its initializer-refined type
• route those locals through js_array_is_array, so the predicate checks the current runtime value
• retain the existing compile-time fold for literals and locals that were never reassigned

Closes #7844

Reproduction

On origin/main (079e646dd), the same six-case program produced stale answers in both directions:

number-to-array false true object
string-to-array false true object
null-to-array false true object
array-to-number true false number
array-to-string true false string
unchanged-array true true object

The second boolean is instanceof Array, showing that the runtime value itself was correct. Node and the fixed compiler both produce:

number-to-array true true object
string-to-array true true object
null-to-array true true object
array-to-number false false number
array-to-string false false string
unchanged-array true true object

The A/B used two compilers against the same prebuilt runtime.

Validation

• cargo test -p perry-codegen --lib: 877 passed
• focused IR regression: passed; exactly two js_array_is_array calls for the two reassigned locals, while the unchanged-array control remains folded
• native_proof_regressions: 262 passed with only typed_f64_receiver_method_clone_raw_loads_after_composed_guards skipped (the current-main typed_f64_receiver_method_clone_raw_loads_after_composed_guards: the guard-failure edge no longer calls $generic — miscompile or intentional collapse? #7506 failure, fixed separately in fix(codegen): coerce declared-only addition results #7851)
• Node/baseline/fixed semantic A/B: fixed output matches Node in all six cases
• python3 scripts/check_test_registration.py: passed
• cargo fmt --all -- --check, git diff --check, and bash scripts/check_file_size.sh: passed

Summary by CodeRabbit

• Bug Fixes
  • Fixed Array.isArray to correctly evaluate local values at runtime after reassignment.
  • Prevented incorrect array classifications when values change between arrays and non-arrays.
  • Preserved optimized handling for unchanged array values.
• Tests
  • Added coverage for reassigned values, instanceof Array, null, primitives, and unchanged arrays.
• Documentation
  • Added a changelog entry describing the corrected behavior.

[coderabbitai]

📝 Walkthrough

Walkthrough

Array.isArray now evaluates reassigned locals at runtime instead of relying on stale initializer types. Regression tests cover transitions between arrays and non-arrays, while unchanged arrays retain compile-time folding.

Changes

Array.isArray reassignment handling

Layer / File(s)	Summary
Runtime predicate for reassigned locals crates/perry-codegen/src/expr/array_methods.rs, crates/perry-codegen/tests/native_proof_regressions.rs	Reassigned locals bypass static array classification and use runtime evaluation. Regression coverage verifies mutable locals and unchanged immutable arrays.
Reassignment behavior coverage test-files/test_gap_7844_array_isarray_reassigned_local.ts, changelog.d/7855-array-isarray-reassignment.md	Tests cover array and non-array reassignment states. The changelog records the corrected behavior.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• PerryTS/perry#7052: Addresses stale compile-time assumptions for reassigned locals.
• PerryTS/perry#7188: Adjusts compile-time array-related folding when static type evidence is insufficient.
• PerryTS/perry#7831: Updates codegen type analysis to avoid unsafe constant folding.

Suggested labels: bug

Suggested reviewers: thehypnoo

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely identifies the main fix: invalidating Array.isArray constant folding after reassignment.
Description check	✅ Passed	The description explains the fix, reproduction, linked issue, and validation, although it uses Validation instead of the template's Test plan heading.
Linked Issues check	✅ Passed	The changes address issue #7844 by routing reassigned locals through runtime Array.isArray checks while preserving valid compile-time folding.
Out of Scope Changes check	✅ Passed	The code, regression tests, changelog entry, and validation changes are directly related to the linked issue and PR objectives.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/7844-array-isarray-reassignment

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[proggeramlug]

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[coderabbitai]

🧹 Nitpick comments (2)

crates/perry-codegen/src/expr/array_methods.rs (1)

97-99: 🚀 Performance & Scalability | 🔵 Trivial | ⚡ Quick win

Make the static-type analysis lazy.

bool::then_some evaluates its argument before the method runs. Therefore, crate::type_analysis::static_type_of(ctx, o) still runs for reassigned locals. Replace it with then(|| ...) so reassigned locals skip this analysis.

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@crates/perry-codegen/src/expr/array_methods.rs` around lines 97 - 99, Update
the conditional static-type lookup around static_type_is_still_valid to use lazy
evaluation, replacing then_some with then so
crate::type_analysis::static_type_of(ctx, o) runs only when the validity flag is
true and is skipped for reassigned locals.

Source: Coding guidelines

test-files/test_gap_7844_array_isarray_reassigned_local.ts (1)

1-2: 🎯 Functional Correctness | 🔵 Trivial | ⚡ Quick win

Add a loop reassignment regression case.

The test covers only straight-line assignments. Add one loop case to verify that reassigned_locals invalidates the Array.isArray static fold for loop writes. Run ./scripts/run_gap_tests.sh with Node 26.5.1 from .node-version.

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@test-files/test_gap_7844_array_isarray_reassigned_local.ts` around lines 1 -
2, Extend the regression test around numberToArray with a loop that reassigns
the local, verifying reassigned_locals prevents Array.isArray static folding for
loop writes. Keep the existing straight-line assignment case, and run
./scripts/run_gap_tests.sh using the Node version specified in .node-version.

Source: Coding guidelines

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@crates/perry-codegen/src/expr/array_methods.rs`:
- Around line 97-99: Update the conditional static-type lookup around
static_type_is_still_valid to use lazy evaluation, replacing then_some with then
so crate::type_analysis::static_type_of(ctx, o) runs only when the validity flag
is true and is skipped for reassigned locals.

In `@test-files/test_gap_7844_array_isarray_reassigned_local.ts`:
- Around line 1-2: Extend the regression test around numberToArray with a loop
that reassigns the local, verifying reassigned_locals prevents Array.isArray
static folding for loop writes. Keep the existing straight-line assignment case,
and run ./scripts/run_gap_tests.sh using the Node version specified in
.node-version.

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 63d339df-704e-4ae7-bfa9-910811af4cf8

📥 Commits

Reviewing files that changed from the base of the PR and between 079e646 and 7506179.

📒 Files selected for processing (4)

• changelog.d/7855-array-isarray-reassignment.md
• crates/perry-codegen/src/expr/array_methods.rs
• crates/perry-codegen/tests/native_proof_regressions.rs
• test-files/test_gap_7844_array_isarray_reassigned_local.ts