[cristianoc]

Known bugs filed as issues: #8953 (number literals compared by spelling), #8954 (Some of a constant @as(undefined) case), #8955 (typed arrays), #8956 (@as tags across a signature), and the existing #8938 (overlapping instance classes).

compiler/ml/UntaggedVariants.md states the rules of untagged variants as they hold on master: 14 numbered rules. Each rule cites the code that implements it, the test that pins it, and the issues that shaped it. The PR changes no compiler behaviour.

• Rules:

  • declaration and representation;
  • @as literals;
  • payload kinds and instance types (the hard-coded type-name list);
  • well-formedness, with its errors;
  • run-time recognition: literal cases first, then the payload checks in their two forms;
  • null and arrays;
  • values outside every case;
  • constant folding;
  • option and @notUndefined;
  • @tag and interfaces.

• Known bugs: five, each reproduced on a fresh build of master with a minimal program and its output.

  • Duplicate @as literals are compared by spelling: @as(1) A | @as(1.0) B is accepted, and B dispatches as A.
  • Some(U), with U declared @as(undefined), compiles to undefined.
  • A File lands in a Blob case declared before it (Untagged variant with Blob and File cases never matches File when Blob is declared first #8938).
  • I8(Uint8Array…) type-checks and matches U8.
  • @as tags of payload cases are not compared across a signature.

  The tests do not pin these as correct.

• Open questions: nine, for a mechanism that recognizes payloads by a declared class or typeof kind, such as Declare the runtime class or typeof of types, and annotate the stdlib #8950. They cover unchecked claims, subclasses, overlaps across kinds, the unchecked branch, realms, interfaces and functors, the implied @notUndefined, abbreviations and globals, each with its related issues.

A PR that changes these rules updates this document, so the change reads as a diff to it.

Test plan

• tests/tests/src/untagged_variants_spec_test.res: 14 tests asserting which case each value lands in. Values pass through an @inline(never) identity so they are not folded.
• super_errors: UntaggedAtMostOneString (the string-case limit) and TaggedDuplicateLiteral. The other rejections cite existing fixtures.
• make test, make checkformat, npx biome check and yarn changelog:check: pass on a fresh build.

🤖 Generated with Claude Code

[github-actions]

Semantic diff: https://cdn.rescript-lang.org/semantic-diff/pr-8952/index.html?v=a60282c3d

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.22%. Comparing base (5de48e6) to head (ccbdf61).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8952   +/-   ##
=======================================
  Coverage   81.21%   81.22%           
=======================================
  Files         457      457           
  Lines       63853    63853           
=======================================
+ Hits        51859    51865    +6     
+ Misses      11994    11988    -6     

see 5 files with indirect coverage changes

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commit: ccbdf61

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[cknitt]

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📝 Code Review	✅ Completed 2026-10-22T18:15:15.593038Z	873cf11	Manual request

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Reviewed commit: 873cf11226

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Semantic Diff

3 commits · Full page

Document the rules of untagged variants, with tests pinning them

_{04e8962 · Cristiano Calcagno with Claude Opus 5.5 · Oct 22, 2026}

Why

The rules of untagged variants (payload kinds, run-time recognition, well-formedness, option and @notUndefined, @as and @tag) are stated once, each with the code that implements it and a test that pins it, so a change to them reads as a diff to this statement.

compiler/ml/UntaggedVariants.md states 14 rules, the known bugs observed on this tree, and the open questions for recognizing payloads by declared class or typeof kind. untagged_variants_spec_test.res asserts which case each value lands in; two super_errors fixtures cover the string-case limit and duplicate literals in a tagged variant.

Keep the non-integral number assertion from folding

_{eb22c7d · Christoph Knittel with Claude Opus 5.5 · Oct 22, 2026}

The Codex review on #8952 found that intOrStringName(raw(1.5)) was constant-folded to "int", so the test passed without dispatching at run time; the value now goes through the non-inlined id first.

Distinguish record payloads in the option table

_{ccbdf61 · Christoph Knittel with Claude Opus 5.5 · Oct 22, 2026}

The Codex review on #8952 found that the rule 12 table said no record holds undefined, while an @unboxed record and a one-field inline record take the answer from their field; the table and rule 12's tests now cover each record shape.