runtime/codegen: evaluate async function default parameters and trailing-comma length semantics

[andrewtdiz]

Summary

Async function default parameters are evaluated in the correct parameter environment, abrupt completions propagate, and .length ignores parameters after the first default/trailing syntax per spec.

Affected Test262 rows

• feat(jsx): closes #277 — link-clean _jsx/_jsxs runtime stubs #297 language/statements/async-function/dflt-params-abrupt.js (runtime-fail: Uncaught exception: Test262Error:)
• fix(doc-tests): closes #244 — auto-downsample retina 2× screenshots before SSIM #298 language/statements/async-function/dflt-params-arg-val-not-undefined.js (diff: (no output))
• Wire getLocale, getAppIcon, ethers.Wallet.createRandom (closes #278) #299 language/statements/async-function/dflt-params-arg-val-undefined.js (diff: (no output))
• Failed to link "user32.lib" #300 language/statements/async-function/dflt-params-ref-later.js (diff: (no output))
• feat: Web Streams API (ReadableStream + WritableStream + TransformStream) — closes #237 #301 language/statements/async-function/dflt-params-ref-prior.js (diff: (no output))
• Map/Set's for...of loop doesn't work for class instance field #302 language/statements/async-function/dflt-params-ref-self.js (diff: (no output))
• feat(platform): Add Windows 7 compatibility support for compiled executables #303 language/statements/async-function/dflt-params-trailing-comma.js (runtime-fail: Uncaught exception: length is properly set Expected SameValue(«2», «1») to be true)
• perry compile silently produces wrong output when import resolves through namespace re-export (export * as X from ...) #310 language/statements/async-function/evaluation-default-that-throws.js (runtime-fail: TypeError: value is not a function)
• perry-codegen: string self-append: local 441 not in scope on Effect's internal/channel.ts #319 language/statements/async-function/params-trailing-comma-multiple.js (diff: (no output))
• perry-stdlib: ReadableStream / Web Streams API not implemented #320 language/statements/async-function/params-trailing-comma-single.js (diff: (no output))

Perry actual behavior from report

• 7 x diff: (no output)
• 1 x runtime-fail: TypeError: value is not a function
• 1 x runtime-fail: Uncaught exception: length is properly set Expected SameValue(«2», «1») to be true
• 1 x runtime-fail: Uncaught exception: Test262Error:

Node / ECMAScript expected behavior

Async function default parameters are evaluated in the correct parameter environment, abrupt completions propagate, and .length ignores parameters after the first default/trailing syntax per spec.

Likely subsystem

HIR parameter environment lowering / async function metadata

Acceptance criteria

• Default parameter reference ordering and abrupt completion rows match Node.
• Async function .length for default/trailing-comma cases matches Test262.
• The async body is not entered after a parameter-initializer throw.

Validation

Run the Test262 differential against the affected area and confirm these rows leave the gap report:

scripts/test262_subset.py --root vendor/test262 --dir language/statements/async-function --sample-cap 99999 --report /tmp/ASYNC_DEFAULT_PARAMS.json

This is Perry language/runtime semantics triage, not a Node core API compatibility issue.

[proggeramlug]

Verified resolved against current main (built fresh from origin/main, 207 commits ahead of an older checkout) and compared byte-for-byte to node --experimental-strip-types. Async default-parameter semantics match node: an abrupt default initializer rejects the promise without entering the body, and length ignores parameters after the first default while counting a trailing comma. Closing as resolved.