Iterator protocol: make GetIterator require and honor @@iterator

[chrisbbreuer]

Parents: #475, #493
Related: #750, #751

Problem

The shared iteratorOf entry point is more permissive than ECMA-262 GetIterator:

• an ordinary object with a .next property but no Symbol.iterator is accepted as iterable;
• generator objects are returned before their observable Symbol.iterator property is read, so an override, getter, deletion, or non-callable value is ignored.

A current-main probe returns false,1,1,0 for [manualObjectThrew, manualNextCalls, overriddenGeneratorSum, iteratorGetterCalls]; the specified result is true,0,7,1. This affects consumers such as Math.sumPrecise even though their subsequent IteratorStep behavior is correct.

Scope

• Make the language GetIterator path perform ordinary GetMethod on @@iterator, require it to be callable, call it once with the input as receiver, and require an Object result.
• Honor observable overrides/deletions on generator and other built-in iterator objects.
• Keep deliberate GetIteratorDirect/GetIteratorFlattenable consumers distinct.
• Audit every iteratorOf caller for the correct abstract operation and retain one captured [[NextMethod]] per iterator record.
• Preserve intrinsic array/string/Map/Set iteration behavior without changing their iterator storage or next implementations.

No-workaround rules

• No .next-property duck typing for GetIterator, generator/builtin bypass, array-only shortcut that skips observable @@iterator, repeated method lookup, source rewriting, or silent fallback after a missing/non-callable method.
• No stack, queue, job, Promise, Worker, or Map/Set iterator implementation changes.

Acceptance

• Plain .next objects are rejected without calling next; an explicit @@iterator makes them iterable.
• Generator/builtin @@iterator getters, overrides, deletion, non-callable values, receiver, and result validation are observed in spec order.
• Every iteratorOf caller is classified as GetIterator, GetIteratorDirect, or GetIteratorFlattenable and captures [[NextMethod]] once where required.
• Focused units, affected Test262 iterator/builtin subtrees, and full integration gates pass with exact nonzero accounting and zero unrelated flips.

[chrisbbreuer]

First staged slice is complete locally: strict GetIterator record construction plus Math.sumPrecise migration. The adversarial probe changes from false,1,1,0 to specified true,0,7,1; focused Math units are 4/4, 0 leaked; exact-parent Test262 Math.sumPrecise is 10 pass -> 10 pass, 0 flips. Commit is intentionally held from main until the current descendant matrix finishes its unaffected gates; that matrix’s only observed failure so far is the separately tracked #722 case.

[chrisbbreuer]

Second staged slice is complete locally: Object.fromEntries now uses strict GetIterator plus its captured [[NextMethod]], rejects object-tagged Symbol/BigInt primitives, and preserves the primary entry TypeError across IteratorClose. Focused regression: 1/1, 0 leaked. Exact-parent Test262 test/built-ins/Object/fromEntries: 25 pass -> 25 pass, 0 flips. Both #752 commits remain held while the existing main matrix finishes its unaffected long-running gates.

[chrisbbreuer]

Third staged slice is complete locally: WebAssembly.Tag parameter and WebAssembly.Exception payload sequences now require @@iterator and reuse captured [[NextMethod]]. Focused Wasm API regression: 1/1, 0 leaked. The checked-out Test262 corpus contains 0 Tag/Exception files; later CI Wasm smoke jobs are the integration evidence. All three commits remain held while the existing main matrix completes.

[chrisbbreuer]

Three strict-GetIterator slices are now on main:

• 8e4d7b90 fix(math): honor sumPrecise iterator method (#752)
• b9861937 fix(object): capture fromEntries iterator record (#752)
• db31991e fix(wasm): require iterable exception sequences (#752)

Exact evidence remains: Math.sumPrecise 10 -> 10 pass, Object.fromEntries 25 -> 25 pass, 0 flips in both; focused regressions 4/4, 1/1, and 1/1 respectively, all with 0 leaked. Main integration: CI 32808460795. The broader caller-classification acceptance item remains open; this intentionally does not touch Promise, Worker, stack/queue/job, or Map/Set iterator implementation paths.

[chrisbbreuer]

Spread syntax is now strict on main in 32ca9e3f fix(spread): require strict iterator records (#752). Array, call, and constructor spread require @@iterator, reuse one captured [[NextMethod]], and no longer direct-copy packed arrays while skipping observable %ArrayIteratorPrototype%.next lookup/calls.

Evidence: exact regression 1/1, complete spread unit filter 13/13, 0 leaked. Exact-parent Test262: array 52 -> 52 pass, call 91 -> 91 pass with the same pre-existing tco-non-eval-with.js failure, and new 59 -> 59 pass; 0 flips across 202 passing cases. Descendant integration: CI 32809472046.

[chrisbbreuer]

Compiled sync iteration is strict on main in e6bfc95e fix(vm): require iterables for iterator bytecode (#752). VM iter_of now performs observable @@iterator acquisition and Object validation, while the compiler remains the single owner of [[NextMethod]] capture for compiled for-of, array destructuring, and yield*—avoiding a double next read.

Evidence: focused required-bytecode regression 1/1; related unit filters for-of 6/6, destructuring 20/20, yield-star 3/3, all 0 leaked. Exact-parent Test262: for-of 751 -> 751, yield 63 -> 63, expression generator destructuring 186 -> 186, statement generator destructuring 186 -> 186; 1,186 pass -> 1,186 pass, 0 flips. Descendant integration: CI 32810113953.

[chrisbbreuer]

Tree-walker sync iteration is strict on main in 84b2b84b fix(interpreter): require iterator records in loops (#752). Synchronous for-of and array destructuring now use one strict Iterator Record; packed-array destructuring no longer skips observable iterator methods. The broader filter exposed and this commit also fixes missing anonymous default-function name inference on the general destructuring path.

Evidence: focused tree-walker regression 1/1; for-of units 6/6; complete destructuring filter 20/20, all 0 leaked. Exact-parent Test262: for-of 751 -> 751 plus five destructuring families 670 -> 670; 1,421 pass -> 1,421 pass, 0 flips. Descendant integration: CI 32811738762.