fix(parser): improve diagnostics for double export

[Boshen]

fixes #15870

[Copilot]

Pull request overview

This PR improves error diagnostics when encountering double export keywords in JavaScript/TypeScript module declarations. The parser now explicitly detects and reports when an export keyword appears twice consecutively, providing better error messages to developers.

Changes:

• Added detection logic for consecutive export keywords in the export statement parser
• Implemented error reporting using the existing modifier_already_seen diagnostic

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Boshen]

• fix(parser): improve diagnostics for double export #17992 👈 (View in Graphite)
• main

---

How to use the Graphite Merge Queue

Add either label to this PR to merge it via the merge queue:

• 0-merge - adds this PR to the back of the merge queue
• hotfix - for urgent hot fixes, skip the queue and merge this PR next

You must have a Graphite account in order to use the merge queue. Sign up using this link.

An organization admin has enabled the Graphite Merge Queue in this repository.

Please do not merge from GitHub as this will restart CI on PRs being processed by the merge queue.

This stack of pull requests is managed by Graphite. Learn more about stacking.

[codspeed-hq]

CodSpeed Performance Report

Merging this PR will not alter performance

_{Comparing 01-14-fix_parser_improve_diagnostics_for_double_export_ (a412103) with main (6ac67bc)}

Summary

✅ 42 untouched benchmarks
⏩ 3 skipped benchmarks¹

Footnotes

1. 3 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[graphite-app]

Merge activity

• Jan 14, 11:23 AM UTC: Boshen added this pull request to the Graphite merge queue.
• Jan 14, 11:29 AM UTC: Merged by the Graphite merge queue.