Resolve export bindings to their canonical module

[andreasrosdal]

Two independent reasons why ResolveExport reports a name as ambiguous when both routes reach the same binding. Each commit stands on its own, but the four test262/test/language/module-code/ambiguous-export-bindings/ tests need both, so they are sent together.

1. Re-export an imported name as an indirect export

import {x} from "m"; export {x}; does not create a binding in the re-exporting module. ParseModule says so: an ExportEntry whose [[LocalName]] is one of the importedBoundNames is not appended to localExportEntries but rewritten into an indirect entry {[[ModuleRequest]]: ie.[[ModuleRequest]], [[ImportName]]: ie.[[ImportName]]}, so ResolveExport walks through to the module the name came from.

add_module_variables() keeps it a local export and resolves it to the closure variable, which makes ResolveExport return the re-exporting module as the binding's [[Module]]:

// m.mjs
export const x = 1;
// r1.mjs
import { x } from './m.mjs';
export { x };
// agg.mjs
export * from './m.mjs';
export * from './r1.mjs';
// main.mjs
import { x } from './agg.mjs';   // SyntaxError: export 'x' ... is ambiguous

V8 resolves this to 1. The rewrite happens where the local exports are resolved, since that is where the import entries are available to map the local name back to (module request, import name). A namespace import goes through the same path and lands on the * import name, which js_resolve_export1() already understands as "the imported module's namespace".

2. Compare resolved bindings by their canonical module

The star-export loop decides whether two routes reach the same binding by comparing the module that held the entry and the entry's local name. For export * as ns from "m" that module is whichever module wrote the re-export, not m, so the same namespace re-exported by two modules looks like two distinct bindings:

// n1.mjs, n2.mjs
export * as ns from './m.mjs';
// agg.mjs
export * from './n1.mjs';
export * from './n2.mjs';
// main.mjs
import { ns } from './agg.mjs';   // SyntaxError: ... 'ns' is ambiguous

ResolveExport returns ResolvedBinding { [[Module]]: importedModule, [[BindingName]]: namespace } for such an entry, so the re-exporting module does not take part in the comparison at all. Map an entry to that canonical pair before comparing.

Testing

test/language/module-code/ambiguous-export-bindings goes from 4/9 failures to 0/9; the four entries are dropped from test262_errors.txt. test/language/module-code, test/language/export and test/language/import show no regressions.

Note that V8 also rejects the second example, so this follows the spec text rather than V8.

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