Keep _-prefixed exotic identifiers escaped, reject \"_\"

[cknitt]

The printer printed escaped identifiers that start with _ bare even when the scanner can't read them back. The scanner treats _ as an identifier only when a letter, digit, _ or $ follows, so these became syntax errors after formatting:

let \"_'" = 2         // printed as `let _' = 2`
let f = (~\"_'") => 1 // printed as `(~_') => 1`
type r = {\"_'": int}  // printed as `{_': int}`
let b = #"_"           // printed as `#_`

classify_ident_content now mirrors the scanner, and editor completion uses the same rule for polyvariant tags.

Breaking: \"_" is now rejected as a quoted identifier ("_ can't be used as a quoted identifier."). It couldn't be told apart from the _ wildcard and the placeholder of underscore apply sugar: f(\"_") was parsed as f(_), i.e. __x => f(__x), and let \"_" = 1 was printed as let _ = 1. Nothing in the repo used it; a JS name _ is still reachable through @as("_") or an external's string name, and the tag #"_" stays valid.

New tests: printer/expr/underscoreExoticIdent.res, parsing/errors/scanner/quotedUnderscore.res, and a #"_" tag in the CompletionExpressions analysis test.

🤖 Generated with Claude Code

https://claude.ai/code/session_01NrGqwcpQ2sF3RiHA48TxmU

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.38%. Comparing base (904027b) to head (b8e5d07).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
analysis/src/utils.ml	0.00%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8795   +/-   ##
=======================================
  Coverage   80.38%   80.38%           
=======================================
  Files         447      447           
  Lines       60979    60991   +12     
=======================================
+ Hits        49016    49026   +10     
- Misses      11963    11965    +2     

Files with missing lines	Coverage Δ
compiler/syntax/src/res_printer.ml	93.53% <100.00%> (+0.01%)	⬆️
compiler/syntax/src/res_scanner.ml	91.16% <100.00%> (+0.06%)	⬆️
analysis/src/utils.ml	50.57% <0.00%> (-0.31%)	⬇️

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[cknitt]

@codex review

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-12T04:50:29.717077Z	b8e5d07	Manual request

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[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep it up!

Reviewed commit: 933ffa5e26

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[cknitt]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Hooray!

Reviewed commit: b8e5d07c44

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

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