feat(typescript-estree): private identifiers can only appear on LHS of in expressions

[Josh-Cena]

PR Checklist

• Addresses an existing open issue: fixes AST tightening: PrivateIdentifier can only be LHS of BinaryExpression[operator='in'] #9222
• That issue was marked as accepting prs
• Steps in Contributing were taken

Overview

[typescript-eslint]

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View your CI Pipeline Execution ↗ for commit 546a854

Command	Status	Duration	Result
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☁️ Nx Cloud last updated this comment at 2025-10-06 12:42:14 UTC

[Josh-Cena]

Converting it to draft, because TS is very wack about private identifiers: console.log(#a) is also valid.

[JoshuaKGoldberg]

👋 just checking in @Josh-Cena, is there anything we should do with this?

[JoshuaKGoldberg]

Chatted 1:1, this is blocked on a resolution to microsoft/TypeScript#58754.

[kirkwaiblinger]

Since the TS fix has been languishing with no progress for over a year, can we just move forward with this on our side? Or do we need to keep this hard-blocked?

[JoshuaKGoldberg]

Yeah, I think we should consider ourselves un-blocked at this point.

[codecov]

Codecov Report

❌ Patch coverage is 23.07692% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.72%. Comparing base (9530e02) to head (546a854).
⚠️ Report is 39 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/typescript-estree/src/convert.ts	23.07%	10 Missing ⚠️

❌ Your patch status has failed because the patch coverage (23.07%) is below the target coverage (90.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9232      +/-   ##
==========================================
- Coverage   90.93%   90.72%   -0.21%     
==========================================
  Files         506      516      +10     
  Lines       51394    52027     +633     
  Branches     8482     8596     +114     
==========================================
+ Hits        46735    47203     +468     
- Misses       4646     4810     +164     
- Partials       13       14       +1     

Flag	Coverage Δ
unittest	90.72% <23.07%> (-0.21%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...s/ast-spec/src/expression/BinaryExpression/spec.ts	100.00% <ø> (ø)
packages/typescript-estree/src/convert.ts	31.12% <23.07%> (+0.89%)	⬆️

... and 30 files with indirect coverage changes

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[JoshuaKGoldberg]

LGTM! I think I'm mildly in favor of @kirkwaiblinger's suggestion, so I'll defer to you two on that. 🙂