TypedArray.prototype.at() returns undefined instead of element value (Int32Array, Float64Array, Uint8Array)

[proggeramlug]

Summary

Tracked in test-parity/known_failures.json as test_gap_array_methods (status: known_limitation). Categorical typed-array gap, same family as test_gap_typed_arrays: Perry's .at() codegen fast path is wired for regular Array but not for the TypedArray instance shapes (Int32Array, Float64Array, Uint8Array, …).

Perry:  Int32Array.at(0): undefined
Node:   Int32Array.at(0): 1

Was passing 26/28 at v0.5.319 after the SSO-unboxing fix transitively flipped it; regressed somewhere in v0.5.319→v0.5.343 (likely an SSO/typed-array interaction in the bulk extraction series).

Minimal repro

const i32 = new Int32Array([1, 2, 3, 4, 5]);
console.log(i32.at(0));   // expected: 1  — actual: undefined
console.log(i32.at(-1));  // expected: 5  — actual: undefined

const f64 = new Float64Array([1.1, 2.2, 3.3]);
console.log(f64.at(-1));  // expected: 3.3  — actual: undefined

const u8 = new Uint8Array([255, 0, 128]);
console.log(u8.at(1));    // expected: 0  — actual: undefined

// (Regular Array.at still works:)
const arr = [10, 20, 30];
console.log(arr.at(-1));  // 30  ✓

Full test: test-files/test_gap_array_methods.ts (the Int32Array.at(0): / Int32Array.at(-1): / Float64Array.at(-1): / Uint8Array.at(1): lines).

How to reproduce

git clone https://github.com/PerryTS/perry && cd perry
cargo build --release -p perry-runtime -p perry-stdlib -p perry

node --experimental-strip-types test-files/test_gap_array_methods.ts > /tmp/node.out
./target/release/perry test-files/test_gap_array_methods.ts -o /tmp/perry_test
/tmp/perry_test > /tmp/perry.out
diff /tmp/node.out /tmp/perry.out

Likely fix area

crates/perry-codegen/src/lower_array_method.rs — the .at(i) arm dispatches against the receiver's HIR Type. The Array variant works; Int32Array / Float64Array / Uint8Array typed-array shapes fall through to a generic byname-property dispatch that returns undefined.

For each TypedArray flavor, the fast path needs to:

1. Look up the receiver's element type and stride (Int32 = 4 bytes, Float64 = 8, Uint8 = 1).
2. Read length from the Uint8Array/Int32Array/Float64Array header.
3. Normalize the index (negative → length+i).
4. Bounds check (returns undefined on out-of-range, like the spec).
5. Direct memory load + NaN-box / int box for the result.

Related

• Was flipped to passing transiently by the v0.5.319 SSO fix; regressed in the v0.5.319→v0.5.343 extraction series. A bisect from v0.5.319 forward might point at the exact commit that broke it.
• test_gap_typed_arrays is the other categorical gap (new Uint8Array(buffer.buffer) and friends — different shape but same crate).

Acceptance

test-files/test_gap_array_methods.ts matches Node byte-for-byte; remove from known_failures.json. Update the stale CLAUDE.md TypeScript-Parity-Status header (currently 26/28; actual is 25/28 with this + console_methods + typed_arrays failing).