test262: async-generator yield* delegation drops .return()/.throw() abrupt resume (~10 cases; core feature)

[proggeramlug]

Async-generator yield* delegation drops .return() / .throw() abrupt resume

Split out of #5703 after a worker confirmed its residual was mis-scoped: ~10 of the 11 cases on #5703 are not the #5662 class-expression regression. They are an async-generator delegation gap that reproduces identically for class statements, plain async function*, and instance methods — the class-expression form is irrelevant. (The language/statements/class copies merely bucket as diff instead of runtime-fail because an async-test failure still exits 0.)

Symptom

A yield*-delegating async generator resumed via .return(v) or .throw(e) never forwards the completion to the delegated iterator's return/throw methods, and can't continue after the resume. Test262 signatures: Cannot read properties of undefined, throw-arg-1, reject reason, Promise incorrectly … in language/expressions/class/async-gen-method-static/yield-star-* (and the statements/class + non-class equivalents).

Root (from the #5703 worker's investigation)

Perry routes .throw()→catch and .return()→finally into linearized generator states, but for async generators crates/perry-transform/.../generator/lower/abrupt.rs::build_async_catch_route_body hard-codes a {value: undefined, done: false} return and has no continuation loop (throw_continuation = None, deliberately). So the delegated iterator's throw/return never fires in spec order, the post-resume yielded value is dropped, and the delegation loop cannot resume. A try/catch desugar prototype made the inner iterator's methods fire in correct order but still dropped the resumed value — confirming the blocker is the async-generator abrupt-resume continuation machinery, a substantial standalone feature.

Scope / relationship

• Blocks ~10 cases on REGRESSION residual: class-expression static-method default-param/destructuring — 11 fails (#5662) #5703 (the setter case there is already fixed by fix(hir): #5703 — apply default-parameter prologue for class setters #5744).
• Overlaps the "async-generator static methods (yield* delegation)" subcluster of test262 language/class tail — 197 fails (async-gen static methods, accessor-name computed-in, private brand) #5592.
• Sync generators handle this; only the async-generator path is missing the continuation.

Measure

scripts/test262_subset.py --root vendor/test262 --dir language/expressions/class/async-gen-method-static language/statements/class/async-gen-method-static --jobs 8

[proggeramlug]

Cross-link from #5591 work: the built-ins/AsyncFromSyncIteratorPrototype return/throw failures share this same root. Per the #5591 worker, the linearized yield* drive loop in crates/perry-transform/src/generator/linearize.rs only delegates next to the inner iterator, not return/throw — the same abrupt-delegation gap as the async-generator yield* cases here. Fixing the abrupt yield* delegation should clear both clusters.

[proggeramlug]

PR #5751 (#5592) lands the .return() half of this: async-gen yield* now forwards gen.return(v) to the delegated iterator + fixes the non-object next() check and AsyncFromSyncIterator GetMethod-once order. The .throw() half remains — its {done:true} branch must resume the outer body after the yield*, which needs the async-generator dispatch-loop continuation that sync generators already have (#4374). So this issue narrows to the throw-forwarding + continuation piece once #5751 merges.

[proggeramlug]

⚠️ Regression watch (re-measure 2026-06-28, main @ 0e89645): the merged #5751 (async-gen yield* return() forwarding + non-object next()) introduced 12 new yield-star-sync-next failures — yield* delegating to a sync iterator's next() now fails across language/{expressions,statements}/async-generator + class async-gen methods. Same machinery as this ticket (generator/lower/abrupt.rs + linearize.rs); fold the sync-next delegation fix in alongside the .throw() half.