[cknitt]

Fixes #8956.

Interface inclusion compared the @as literal of nullary cases only. An implementation could swap the tags of two cases with payloads, and its clients then matched M.A(_) as C:

module M: {
  type t = A({x: int}) | C({x: int})
  let a: t
} = {
  type t = | @as("C") A({x: int}) | @as("A") C({x: int})
  let a = A({x: 1})
}
Console.log(switch M.a { | M.A(_) => "A" | M.C(_) => "C" })   // C

Includecore.compare_variants now compares the tags of tagged cases with a payload as it does those of nullary cases. An untagged case's @as does not change its representation (spec rule 2) and is not compared. Type declarations in messages now print the @as of tagged cases with a payload too (printtyp.ml:tree_of_constructor); before, both sides of this mismatch printed as A({x: int}) | C({x: int}):

Type declarations do not match:
  type t = @as("C") A({x: int}) | @as("A") C({x: int})
is not included in
  type t = A({x: int}) | C({x: int})
The internal representations for case A are not equal.

UntaggedVariants.md: rule 14 states the comparison, and the known bug is removed.

New test: super_errors/variant_payload_tag_mismatch.

🤖 Generated with Claude Code

https://claude.ai/code/session_01CMkrYaktctdKc4M41WF36t

[github-actions]

Changelog

• 🐛 Reject an implementation whose variant cases with a payload have other @as tags than the interface states, and print those tags in type declarations.

Semantic Diff

1 commit · Full page

Compare the @as tags of variant cases with payloads across signatures

_{ed8d4a4 · cknitt + Claude · Oct 24, 2026, 06:14 UTC}

Why

Interface inclusion compared the @as literal of nullary cases only. An implementation could swap the tags of two cases with payloads, and its clients then matched M.A(_) as C (#8956).

The tags of tagged cases with a payload are compared as those of nullary cases are. An untagged case's @as does not change its representation and is not compared. Type declarations in messages now print the @as of a tagged case with a payload too, so the mismatch shows.

Diagnostic added

module M: {
  type t = A({x: int}) | C({x: int})
} = {
  type t = | @as("C") A({x: int}) | @as("A") C({x: int})
}

diagnostic · After

Type declarations do not match:
  type t = @as("C") A({x: int}) | @as("A") C({x: int})
is not included in
  type t = A({x: int}) | C({x: int})
The internal representations for case A are not equal.

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.29%. Comparing base (d8db3c6) to head (ed8d4a4).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
compiler/ml/printtyp.ml	80.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9009   +/-   ##
=======================================
  Coverage   81.29%   81.29%           
=======================================
  Files         456      456           
  Lines       63836    63843    +7     
=======================================
+ Hits        51893    51899    +6     
- Misses      11943    11944    +1     

Files with missing lines	Coverage Δ
compiler/ml/includecore.ml	78.46% <100.00%> (+0.42%)	⬆️
compiler/ml/printtyp.ml	70.55% <80.00%> (-0.05%)	⬇️

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commit: ed8d4a4

[github-actions]

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[cknitt]

@codex review

[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 2026-10-24T06:40:35.222592Z	ed8d4a4	Manual request

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[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. More of your lovely PRs please.

Reviewed commit: ed8d4a46c8

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