VM: preserve Reference semantics for parameter assignment and update expressions

[chrisbbreuer]

Parent: #465. Related: #798, #771.

Verified remaining boundary

The #798 exact-parent comparison (8a1398f, pinned Test262 4249661388e5d3f92a85186213da140a6481490f) still rejects the eight ordinary arrow/function/object-method/class-method variants of dflt-params-arg-val-not-undefined.js at the parameter-prologue admission boundary. Their initializers use compound assignments such as falseCount += 1. All eight fail required bytecode both before and after #798; the corresponding default-tier diagnostics pass. This is not a regression from identifier admission.

The compiler has existing exact Reference lowering for assignment, compound/logical assignment, and prefix/postfix updates, but the parameter classifier only admits plain identifier assignment. Reuse the canonical checked local/upvalue/Environment/global and member Reference paths where their complete parameter semantics are proven; do not rewrite compound operations into separate name lookups.

Acceptance

• Admit fully lowered assignment, compound/logical assignment, and update expressions in parameter initializers and computed binding keys, while retaining explicit rejection for genuinely unsupported nested syntax.
• Preserve self/later-formal TDZ, const/strict errors, parameter/body separation, dynamic with/unscopables Reference selection, and nested/escaping lexical identity.
• Member bases and computed keys evaluate exactly once; getter/RHS/coercion/setter ordering, logical short-circuiting, named evaluation, and abrupt completion match the tree-walker.
• Provided arguments suppress every initializer effect. Add executed-default controls as well: the eight existing provided-argument files alone do not establish assignment execution semantics.
• Paired ordinary/arrow/method/nested/destructuring witnesses, compiler OOM, no-GIL/TSan, full units, and exact before/after Test262 accounting pass.
• Land an issue-linked conventional commit; report required-tier gains separately from default-tier flips and pushed CI.

No AST callback, repeated reference resolution, binding-value snapshot, skipped TDZ/const check, policy fallback, or changes to native stack/queue/job, Promise/Worker lifecycle, or Map/Set iterator implementations. Object/array/class literal admission and other initializer forms are outside this focused child.

Landed implementation and evidence

e4f76af6 — feat(vm): admit parameter assignment expressions (#799) is on origin/main, authored and committed by Chris without trailers. Numeric prerequisite d2ad8a8f (#800) fixes the independently discovered Symbol and wide-BigInt update defects.

The classifier uses canonical parameter-phase binding plans and existing activation-owned member/super temporaries. It does not introduce a callback, duplicated resolution, or new native ownership model. Unsupported nested object/array/class literals and optional forms remain explicit boundaries.

Measured against 2865b8b and Test262 4249661388e5d3f92a85186213da140a6481490f:

• Same 9,643 eligible positive files: 8,809 pass / 834 fail -> 8,838 pass / 805 fail: 29 required-bytecode gains, zero regressions.
• Gains: expressions/arrow-function 4, expressions/class 8, expressions/function 4, expressions/object/method-definition 1, statements/class 8, statements/function 4.
• All 13 default-tier reports are byte-identical: 11,494 pass / 0 fail; 0 default-tier flips. The 805 remaining paired failures are not excluded or claimed implemented.
• Original 39 assertion fixtures: 0 -> 39 paired passes; four additional numeric prerequisite controls bring the final matrix to 43 passing paired cases. Covers real executed defaults, provided arguments, every compound operator, logical effects/naming, TDZ/const/strict, with/unscopables, closures, computed keys, reentry and super.
• Final Debug and ReleaseSafe+TSan assignment filters each pass 3/3, including shared no-GIL invocation isolation and compiler allocation replay. Stable admission guard also passes under TSan; zero reports/failures/leaks.
• Final combined unit suite: 2,026 pass, 1 platform skip, 0 failures/leaks, 350.5 seconds across four timing-balanced shards. Seed-1 fuzz: 400 default + 300 verify programs, zero failures. Format/diff checks pass.

The final combined comparison has exactly the same gains and regressions as the #799-only candidate. No public performance claim. Broader borrowed-state GC failures found during review are tracked, with actual relocation reproductions, in #801; no native stack/queue/job or lifecycle implementation was changed.

• Pushed required integration verified: CI 33130214332 at descendant e4f76af completed successfully: 53 jobs passed, one scheduled-only job skipped, zero failures. Superseded/cancelled earlier runs are not counted as successful gates.

Evidence: /Users/chris/.cache/zig-js-799.NeLWpV/README.md. All 59 temporary #799/#800 corpus fixtures were byte-compared and removed; unit assertions and retained fixtures remain. Generated isolated caches were cleaned (1.4 GB -> 144 MB) while preserving logs, hashes, source diffs, final binaries and shard timing plans. User-owned .codex was untouched.

[chrisbbreuer]

The identifier-reference prerequisite landed as 2865b8b4 (#798). Its exact before/after comparison retains the eight compound-assignment provided-argument failures named here. Use this commit as the next implementation baseline; #798's 334 other admission gains are not this issue's results.

[chrisbbreuer]

Implementation and final focused checks now pass. The parameter matrix is expanded to 43 paired cases, including Symbol and wide/overflow BigInt controls enabled by prerequisite #800. Shared no-GIL invocation isolation and compiler allocation replay pass; final ReleaseSafe+TSan filters report 3 parameter-assignment tests and the stable-admission guard passing without reports/leaks. The final combined 9,643-file inventory is byte-identical to the baseline inventory: 8,809 pass / 834 fail -> 8,838 pass / 805 fail, +29 / zero regressions. All 13 default-tier reports remain byte-identical (11,494 pass / 0 fail). The #799-only full suite passed 2023 + 1 skip; the final combined runtime suite and fuzz checks are now running before landing. No native stack/queue/job or lifecycle redesign.

[chrisbbreuer]

Closing after the linked implementation and full pushed integration were verified: CI 33130214332 passed all 53 executed jobs (one scheduled-only skip). The issue body records exact conformance denominators, local validation and remaining independent follow-ups.