test262: Iterator conformance fixes (wave 13) - #75
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A generator used in expression position — most commonly the immediately-invoked
form `(function*(){ … })()` that test262 uses to build a throwaway iterator —
was lowered as an ordinary function: its `is_generator` flag was only honoured
for function DECLARATIONS (in as_function_decl), so the expression body was never
rewritten to the resumable state machine. The call therefore returned `undefined`
(empty body) or raised "yield in an unsupported generator shape", and any later
`.next()`/iterator-helper call on the result threw a TypeError.
Rewrite a generator function expression in the lambda pre-pass exactly as a
declaration is rewritten, falling back to the untransformed body on an unsupported
shape so the surviving `yield` still reports a clean Unsupported error. This makes
`(function*(){…})()` produce a real generator, fixing the Iterator-helper
"already exhausted" cases (some/every/find/reduce/toArray) and letting the lazy
helpers compose over generator-expression sources.
Iterator-helper tests routinely close a source with `iterator.return()` before
building a helper over it; `.return` was undefined, so the call raised
{badfun,undefined}. Add gen_return across the three runtime layers (FFI impl +
grouped export, emit_core resolve_js allow-list, rt_js @external) and route
`x.return(v)` in the generator dispatch cluster.
Per §27.5.1.4 it closes the generator and returns {value: v, done: true} (v
defaults to undefined); a non-generator receiver delegates to a user `return`
method. Closing swaps the step for a permanently-done step while keeping the
{js_gen,_} tag, so the iterator helpers (map/filter/take/drop/…), which dispatch
on that tag, correctly see the closed source as an empty iterator.
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Raises Iterator/IteratorHelperPrototype test262 conformance from 42 → 60 passing (+18; RAN 74 → 92) via two generator-runtime fixes.
1. Transform generator function expressions (not just declarations)
is_generatorwas only honoured for function DECLARATIONS, so(function*(){ … })()— the immediately-invoked form test262 uses to build a throwaway iterator — lowered as a plain function, returningundefined(or raising "yield in an unsupported generator shape"). The lambda pre-pass now rewrites a generator expression body to the same resumable state machine, falling back to the untransformed body on an unsupported shape. Fixes the "iterator-already-exhausted" cases (some/every/find/reduce/toArray). (+10)2.
Generator.prototype.return(§27.5.1.4)iterator.return()was undefined →{badfun,undefined}. Addedgen_returnacross the FFI (grouped export next togen_next),emit_coreallow-list, andrt_jsexternal, routed in the generator dispatch cluster. Closes the generator and returns{value: v, done: true}; a non-generator receiver delegates to a userreturnmethod. Closing swaps in a permanently-done step while keeping the{js_gen,_}tag, so the lazy helpers (map/filter/take/drop) see the closed source as empty. Fixes the "underlying-iterator-closed" cluster. (+8)Spec-driven tests appended at EOF under
// ==== Iterator (wave 13) ====. js_compiler_test fully green (only the 3 pre-existing WASM .wat fixture failures remain); erlc/gleam build/format clean.