[cknitt]

Fixes #8954.

Some(U) with @as(undefined) U compiled to plain undefined when U was a constant, so it equalled None, while Some(v) with v = U at run time stayed distinct:

@unboxed type u = | @as(undefined) U | V(string)
let x: option<u> = Some(U)                       // let x = undefined;
@inline(never) let mk = (v: u): option<u> => Some(v)
Console.log2(x == None, mk(U) == None)           // true false

The cause was the one the spec conjectured: Lambda.const_some_none_depth treated every Const_constructor as "not a none", including one whose literal is undefined. Such a constructor is now a none, so Some of it is boxed like Some(None): let x = Primitive_option.some(undefined);, and the program prints false false. Tagged variants with an @as(undefined) case are fixed the same way.

UntaggedVariants.md: the known bug is removed, and rule 12 states the case.

New test: untagged_variants_spec_test.res rule 12 checks a constant Some(WUndefined).

🤖 Generated with Claude Code

https://claude.ai/code/session_01CMkrYaktctdKc4M41WF36t

[github-actions]

Changelog

• 🐛 Keep Some(U) distinct from None when U is a constant case represented by undefined (@as(undefined)).

Semantic Diff

1 commit · Full page

Box Some of a constant case represented by undefined

_{b5836de · cknitt + Claude · Oct 24, 2026, 06:10 UTC}

Why

Some(U) with @as(undefined) U compiled to plain undefined when U was a constant, so it was None, while Some(v) with v = U passed at run time stayed distinct (#8954).

A constant constructor whose literal is undefined is now a none for const_some_none_depth, so Some of it is boxed like Some(None). This holds for untagged and tagged variants alike. The spec's known bug is gone, and rule 12 states the case.

JS modified

@unboxed type u = | @as(undefined) U | V(string)
let x: option<u> = Some(U)

js

-let x = undefined;
+let x = Primitive_option.some(undefined);

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.29%. Comparing base (d8db3c6) to head (b5836de).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
compiler/ml/lambda.ml	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9008   +/-   ##
=======================================
  Coverage   81.29%   81.29%           
=======================================
  Files         456      456           
  Lines       63836    63837    +1     
=======================================
+ Hits        51893    51894    +1     
  Misses      11943    11943           

Files with missing lines	Coverage Δ
compiler/ml/lambda.ml	68.08% <50.00%> (+0.06%)	⬆️

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commit: b5836de

[cknitt]

@codex review

[github-actions]

Developer playground preview: https://rescript-lang.github.io/rescript/dev-playground/?version=pr-9008

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-24T06:36:16.533676Z	b5836de	Manual request

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[chatgpt-codex-connector]

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

Reviewed commit: b5836debf5

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