[cknitt]

Fixes #9124.

A private type's values can only be built in the module that defines it, but structural coercion built them anywhere. Each of these compiled before:

module Blob: { type t = private {size: float} } = { type t = {size: float} }
type plain = {size: float}
let fake = (p: plain) => (p :> Blob.t)            // a plain record
let forged = (Fake.make(1.) :> Blob.t)            // another module's private record

module Color: { type t = private Red | Green } = { type t = Red | Green }
let c1 = (Red: plain :> Color.t)                  // a plain variant
let c2 = (#Red: [#Red] :> Color.t)                // a polymorphic variant
let i = ("a" :> Id.t)                             // a string, into a private @unboxed variant

subtype_rec now refuses a coercion whose target's definition is a private record or variant, unless the source is that same type. One guard in front of the structural branches covers all routes. The new error says:

Blob.t is private: its values can only be built in the module that defines it, so another type cannot be coerced to it.

Coercing out of a private type is unchanged ((b :> plainBlob), (c :> string)).

This is a prerequisite for @instanceOf on private records (#9058), where a forged value would be dispatched as an instance of a class it isn't. It also means the upcast between two private records, e.g. (file :> blob), is refused until the RFC's @extends declares that relationship.

New tests:

• super_errors/fixtures/coercion_into_private_*.res: one per route above.
• RecordCoercion.res and VariantCoercion.res: coercions out of a private record and variant.

🤖 Generated with Claude Code

https://claude.ai/code/session_01CMkrYaktctdKc4M41WF36t

[github-actions]

Changelog

💥 Breaking Change

• Coercing another type into a private record or variant is an error: only the module defining a private type can build its values. Coercing out of a private type still works.

_{💥 A changelog entry is a breaking change, but no commit says what it breaks (Breaking:).}

Semantic Diff

1 commit · Full page

✏️ Refuse coercing another type into a private record or variant

_{052d3cc · cknitt + Claude · Oct 29, 2026, 10:11 UTC · edited}

Why

A private type's values can only be built in the module that defines it, but structural coercion built them anywhere: a plain record, another module's private record with the same fields, a variant, a polymorphic variant or a string coerced into a private record or variant (#9124).

subtype_rec refuses a coercion whose target's definition is a private record or variant unless the source is that same type. Coercing out of a private type is unchanged.

Typing modified (coercion into a private type)

module Blob: { type t = private {size: float} } = { type t = {size: float} }
type plain = {size: float}
let fake = (p: plain) => (p :> Blob.t)

rule · Before: a record coerces into a record with compatible fields, · After: a record or variant coerces into a private record or variant

Before	After
$$\begin{array}{l}\color{red}{\mathrm{private}\ \mathrm{or}\ \mathrm{not}}\end{array}$$	$$\begin{array}{l}\color{green}{\mathrm{only}\ \mathrm{from}\ \mathrm{that}\ \mathrm{same}\ \mathrm{type}}\end{array}$$

Diagnostic added (coercion into a private type)

let fake = (p: plain) => (p :> Blob.t)

diagnostic

-none
+Type plain is not a subtype of Blob.t
+Blob.t is private: its values can only be built in the module that
+defines it, so another type cannot be coerced to it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.48%. Comparing base (799a82f) to head (052d3cc).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9125   +/-   ##
=======================================
  Coverage   81.48%   81.48%           
=======================================
  Files         460      460           
  Lines       64728    64740   +12     
=======================================
+ Hits        52744    52756   +12     
  Misses      11984    11984           

Files with missing lines	Coverage Δ
compiler/ml/ctype.ml	67.52% <100.00%> (+0.14%)	⬆️
compiler/ml/printtyp.ml	71.94% <100.00%> (+0.05%)	⬆️

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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 2026-10-29T13:56:39.148822Z	052d3cc	Manual request

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commit: 052d3cc

[github-actions]

Developer playground preview: https://rescript-lang.github.io/rescript/dev-playground/?version=pr-9125

[cknitt]

@codex review

[chatgpt-codex-connector]

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

Reviewed commit: 052d3cca9e

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[cristianoc]

Looks good to me. Thanks for the changes.