Sep 4, 2025 · Sep 4, 2025 · Sep 4, 2025 · Sep 4, 2025 · Sep 5, 2025 · Sep 5, 2025
diff --git a/AGENTS.md b/AGENTS.md
 #CLAUDE.md
 #AGENTS.md

 This file provides guidance toClaude Code (claude.ai/code) whenworkingwith codein this repository.
 Shared guidance for code agents (Claude, Gemini, Codex, etc.)working in this repository. The files `CLAUDE.md` and `GEMINI.md` are symlinks to this document. For Codex‑specific workflow details also see `CODEX.md`.

 ## Project Overview

 - Avoid introducing meaningless symbols
 - Maintain readable JavaScript output
 - Consider compilation speed impact
 - Use appropriate optimization passes in the Lambda and JS IRs
 - Use appropriate optimization passes in the Lambda and JS IRs

 ## Agent Notes and Gotchas

 - **Node version features:** Runtime docstring tests conditionally enable features by Node major version:
  - 20+: array `toReversed`/`toSorted`
  - 22+: new `Set` APIs and `Promise.withResolvers`
  - 24+: `RegExp.escape`
  Tests auto‑skip unsupported features based on `process.version`.
 - **CPU count in sandboxes:** Some CI/sandboxed environments report `os.cpus().length === 0`.
  - Clamp concurrency/batch size to at least 1 when using `os.cpus()`.
  - This pattern is used in `tests/docstring_tests/DocTest.res` and `cli/rescript-legacy/format.js`.
 - **Formatting in tests:** `make test` checks formatting (OCaml, ReScript, JS, Rust). If it fails locally, run `make format` and re‑run tests.
 - **Executables location:** Build copies platform binaries into `packages/@rescript/<platform>/bin/` and convenience folders like `darwinarm64/`.
 - **Direct dune usage:** You can use `dune build`/`dune build -w`, but prefer `make` targets which also copy executables.

 ## References

 - `CODEX.md`: detailed setup, build, testing, and workflows for agented development
 - `README.md`: high‑level repo overview and usage
 - `Makefile`: authoritative list of build/test targets
diff --git a/CHANGELOG.md b/CHANGELOG.md
 - Fix generation of interfaces for module types containing multiple type constraints. https://github.com/rescript-lang/rescript/pull/7825
 - JSX preserve mode: fix "make is not a valid component name". https://github.com/rescript-lang/rescript/pull/7831
 - Rewatch: include parser arguments of experimental features. https://github.com/rescript-lang/rescript/pull/7836
 - Suppress spurious “constructor … is never used” warnings when constructors are introduced via variant coercions; still warn for constructors only present on the target side. https://github.com/rescript-lang/rescript/pull/7839

 #### :memo: Documentation

diff --git a/compiler/ml/typecore.ml b/compiler/ml/typecore.ml
           raise (Error (loc, env, Not_subtype (tr1, tr2, ctx))));
      (arg, ty', cty')
    in
    (* After a successful coercion, if we coerced between concrete variant
       types, mark the intersection of constructors as positively used for the
       target type to avoid spurious "constructor ... is never used" warnings
       when values are introduced via coercions. *)
    (try
       let _p_src, _p_src_conc, src_decl =
         Ctype.extract_concrete_typedecl env arg.exp_type
       in
       let p_tgt, p_tgt_conc, tgt_decl =
         Ctype.extract_concrete_typedecl env ty'
       in
       match (src_decl.type_kind, tgt_decl.type_kind) with
       | Type_variant src_cons, Type_variant tgt_cons ->
         let module StringSet = Set.Make (String) in
         let src_set =
           List.fold_left
             (fun acc (c : Types.constructor_declaration) ->
               StringSet.add (Ident.name c.cd_id) acc)
             StringSet.empty src_cons
         in
         let has_src name = StringSet.mem name src_set in
         let tgt_ty_name_conc = Path.last p_tgt_conc in
         (* Mark usage for the concrete target decl (implementation view). *)
         List.iter
           (fun (c : Types.constructor_declaration) ->
             let cname = Ident.name c.cd_id in
             if has_src cname then
               Env.mark_constructor_used Env.Positive env tgt_ty_name_conc
                 tgt_decl cname)
           tgt_cons;
         (* If the target type path differs from its concrete decl (e.g.,
            when a signature exposes a private or abstract alias), also mark
            usage on the exposed target declaration so scheduled warnings
            attached to that declaration are cleared. *)
         if not (Path.same p_tgt p_tgt_conc) then
           let exposed_ty_name = Path.last p_tgt in
           let exposed_decl = Env.find_type p_tgt env in
           List.iter
             (fun (c : Types.constructor_declaration) ->
               let cname = Ident.name c.cd_id in
               if has_src cname then
                 Env.mark_constructor_used Env.Positive env exposed_ty_name
                   exposed_decl cname)
             tgt_cons
       | _ -> ()
     with Not_found -> ());
    rue
      {
        exp_desc = arg.exp_desc;
diff --git a/example.res b/example.res
 type a = A | B

 // b spreads a and adds one more constructor
 type b =
  | ...a
  | C

 let upcast = (x: a): b => (x :> b)

 let _ = upcast(A)
diff --git a/issue.txt b/issue.txt
 found a compiler bug: https://github.com/rescript-lang/rescript/issues/7838 (due to the coercing between variants)
 GitHub
 Incorrect compiler warning (constructor is never used to build valu...
 https://rescript-lang.org/try?version=v11.1.4&amp;module=esmodule&amp;code=LYewJgrgNgpgBAUQG4wHYBc4F44G8BQcc6AngA7yY4A+iKGAgvoXLJgBYCGqYs2cAChj10ALmIBKbAD48LIgGcA7gEt0AY3ZxhaTASJFayXQxmCJ8uAF8WNm-l...
 Incorrect compiler warning (constructor is never used to build valu...
 [09:34]Gabriel Nordeborn: What is wrong here...?
 [09:35]jaap: The warning?
 [09:36]jaap: The variants are constructed in handle
 [09:37]jaap: I mean, the coercion constructs the variants
 [09:38]jaap: so if we have variantA :> variantB, we can mark that variantB is always constructed
 [09:38]Gabriel Nordeborn: Right, so if you adapt the example to not use coercion, it marks them as constructed?
 [09:39]Gabriel Nordeborn: It's good if you write this in the PR issue
 [09:39]Gabriel Nordeborn: Right now, just looking at the example, I did not understand what you're referring to
 [09:41]jaap: Yes
 [09:41]jaap: https://rescript-lang.org/try?version=v11.1.4&module=esmodule&code=LYewJgrgNgpgBAUQG4wHYBc4F44G8BQcc6AngA7yY4A+iKGAgvoXLJgBYCGqYs2cAChj10ALmIBKbAD48LIgGcA7gEt0AY3ZxhaTASJFayXQxmCJ8uAF8WNm-lCQ+AZRgAnFG+MYA4iHDi+qwwmGgAjhAwkeLe6AB0VLIQqGr4VvxBpBTE2Ja0cQWxCcxEbHBcPHw4nAokqOqCOhji6FJYskGKqhpaTXp5dCZmAhYG1rYsZeGRkfxCIjEiCW2yFbwwAspqmtoicoaDjGaxTERWEgC00ioA5qggbjBpzI7Q8K4e7rEAQpxggZMQtpUBEojBFroEmZkql0jhMuRKLkxvlCkt0CVghxuOt+DU6g15roWit9gYtj1droyQcTsNRgYbGdAaEQTN4Dgic1DvFWmY1rAuZhRLJWldbvdHs8gA
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 [09:41]jaap: here with two examples
 [09:41]jaap: also updated the ticket
 [09:44]cristianoc: The type system was created with no coercion in mind.
 [09:45]cristianoc: So fix warning —-> compiler error?
 [09:47]Gabriel Nordeborn: @jaap is this not a better example? https://rescript-lang.org/try?version=v11.1.4&module=esmodule&code=LYewJgrgNgpgBAUQG4wHYBc4F44G8BQcc6AngA7yY7JroCC+hcsmAFgIapizZwAUMFBgBcxAJTYAfHiZEAzgHcAlugDGrOINoyiRAD6Ih9KfzGy4AXyZWr+UJB4BlGACcULmhgDiIcKIJEpBTEvAYAdBGe6GHojEQsmqgAjhAwqaJU0hCoKvgWvAHE5JShcBFhUTFxzDBsnNzwOOxyJKiq-Foi4iaF8spqGp2YvXAGUXQmfGa6ltZMCWgpaY0dRhkSWNIcXLACRmIAtJJKAOaoIC4weYz20PDObq5RAELsYP7ztYlL6Ya0MSZsrl8jhCkESjhwpEjFVPnUditmq12ntaOseuZFCp1JojDpdGMjBNNqZzFYiOSaphFqlUrxUV1KugNlt6rsGZhhNJmYdjmcLlcrEA
 ReScript Documentation
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 Try ReScript in the browser
 ReScript Playground
 [09:47]Gabriel Nordeborn: Using the switch fixes the warning of course because you're now constructing it explicitly
 [09:47]Gabriel Nordeborn: If you were to add the switch instead of coercion that'd work as well
 [09:48]Gabriel Nordeborn: But from what I understand the problem is that there's a value of a type that gets coerced to another type, and that other type is not marked as used
 [09:49]Gabriel Nordeborn: The value that's not coerced is marked, but the new type from the coercion is not marked
 [10:35]jaap: @cristianoc there is no way to fix this warning - you need to convert it to a switch expression, but that is a lot of redudant code potentially
 [10:36]cristianoc: That’s actually better. So the message is at least not inconsistent.
 [10:36]jaap: but the tricky part is that when coercing, the members that are in variantA need to marked as used, but not the new ones
 [10:36]cristianoc: I was worried: you remove the case, then get a compile error
 [10:37]jaap: Yes you do get an error - so it's not swallowing that
 [10:37]jaap: if you include it (warning) if you remove it (error)
 [10:38]cristianoc: There’s no such thing in the compiler. As ocaml does not have that. So it will be a genuine extension to handle this.
 And I don’t even know if possible.
 Is this  global property? If so the. Compiler can’t do it.
 [10:38]jaap: but yeah seems to be a gap in the type system after adding the nice spreading of variants and coercing
 [10:39]jaap: No it's local - in the example you see the module signature added, so it only happens if you make the type private, and then coerce it
 [10:40]jaap: I guess there is some mechanism to mark members as constructed - and coercion is not doing that
 [10:40]cristianoc: Ok this is enough discussion.
 Paste it in a prompt and see if it gets close.
diff --git a/tests/build_tests/super_errors/expected/VariantCoercionWarnOnC.res.expected b/tests/build_tests/super_errors/expected/VariantCoercionWarnOnC.res.expected

  [1;33mWarning number 37[0m
  [36m/.../fixtures/VariantCoercionWarnOnC.res[0m:[2m9:3-11:7[0m

   7 [2m│[0m   let log: a => unit
   8 [2m│[0m } = {
   [1;33m9[0m [2m│[0m   [1;33mtype b =[0m
  [1;33m10[0m [2m│[0m [1;33m    | ...a[0m
  [1;33m11[0m [2m│[0m [1;33m    | C[0m
  12 [2m│[0m
  13 [2m│[0m   let log = (x: a) => Js.log((x :> b))

  unused constructor C.
diff --git a/tests/build_tests/super_errors/fixtures/VariantCoercionWarnOnC.res b/tests/build_tests/super_errors/fixtures/VariantCoercionWarnOnC.res
 // Ensure unused-constructor warning still fires for new members introduced
 // only on the target side of a coercion.

 type a = A | B

 module M: {
  let log: a => unit
 } = {
  type b =
    | ...a
    | C

  let log = (x: a) => Js.log((x :> b))
 }

 let _ = M.log(A)
diff --git a/tests/docstring_tests/DocTest.res b/tests/docstring_tests/DocTest.res
  }
 }

 let batchSize = OS.cpus()->Array.length
 // Some environments may report 0 CPUs; clamp to at least 1 to avoid zero-sized batches
 let batchSize = switch OS.cpus()->Array.length {
 | n if n > 0 => n
 | _ => 1
 }

 let runtimePath = Path.join(["packages", "@rescript", "runtime"])

diff --git a/tests/docstring_tests/DocTest.res.js b/tests/docstring_tests/DocTest.res.js
diff --git a/tests/tests/src/VariantCoercionConstructUsed.mjs b/tests/tests/src/VariantCoercionConstructUsed.mjs
 // Generated by ReScript, PLEASE EDIT WITH CARE


 function log(x) {
  console.log(x);
 }

 let DoNotWarn = {
  log: log
 };

 console.log("A");

 export {
  DoNotWarn,
 }
 /*  Not a pure module */
diff --git a/tests/tests/src/VariantCoercionConstructUsed.res b/tests/tests/src/VariantCoercionConstructUsed.res
 // Repro for incorrect "constructor ... is never used" warning with coercions
 // This should compile cleanly without warnings when coercing from a -> b.

 type a = A | B
 module DoNotWarn: {
  let log: a => unit
 } = {
  type b =
    | ...a
    | C

  let log = (x: a) => Js.log((x :> b))
 }

 let _ = DoNotWarn.log(A)
Original file line number	Diff line number	Diff line change
		@@ -1,6 +1,6 @@
		#CLAUDE.md
		#AGENTS.md

		This file provides guidance toClaude Code (claude.ai/code) whenworkingwith codein this repository.
		Shared guidance for code agents (Claude, Gemini, Codex, etc.)working in this repository. The files `CLAUDE.md` and `GEMINI.md` are symlinks to this document. For Codex‑specific workflow details also see `CODEX.md`.

		## Project Overview

Expand DownExpand Up		@@ -136,4 +136,24 @@ The compiler is designed for fast feedback loops and scales to large codebases.
		- Avoid introducing meaningless symbols
		- Maintain readable JavaScript output
		- Consider compilation speed impact
		- Use appropriate optimization passes in the Lambda and JS IRs
		- Use appropriate optimization passes in the Lambda and JS IRs

		## Agent Notes and Gotchas

		- Node version features: Runtime docstring tests conditionally enable features by Node major version:
		- 20+: array `toReversed`/`toSorted`
		- 22+: new `Set` APIs and `Promise.withResolvers`
		- 24+: `RegExp.escape`
		Tests auto‑skip unsupported features based on `process.version`.
		- CPU count in sandboxes: Some CI/sandboxed environments report `os.cpus().length === 0`.
		- Clamp concurrency/batch size to at least 1 when using `os.cpus()`.
		- This pattern is used in `tests/docstring_tests/DocTest.res` and `cli/rescript-legacy/format.js`.
		- Formatting in tests: `make test` checks formatting (OCaml, ReScript, JS, Rust). If it fails locally, run `make format` and re‑run tests.
		- Executables location: Build copies platform binaries into `packages/@rescript/<platform>/bin/` and convenience folders like `darwinarm64/`.
		- Direct dune usage: You can use `dune build`/`dune build -w`, but prefer `make` targets which also copy executables.

		## References

		- `CODEX.md`: detailed setup, build, testing, and workflows for agented development
		- `README.md`: high‑level repo overview and usage
		- `Makefile`: authoritative list of build/test targets
Original file line number	Diff line number	Diff line change
Expand Up		@@ -25,6 +25,7 @@
		- Fix generation of interfaces for module types containing multiple type constraints. https://github.com/rescript-lang/rescript/pull/7825
		- JSX preserve mode: fix "make is not a valid component name". https://github.com/rescript-lang/rescript/pull/7831
		- Rewatch: include parser arguments of experimental features. https://github.com/rescript-lang/rescript/pull/7836
		- Suppress spurious “constructor … is never used” warnings when constructors are introduced via variant coercions; still warn for constructors only present on the target side. https://github.com/rescript-lang/rescript/pull/7839

		#### :memo: Documentation

Expand Down
Original file line number	Diff line number	Diff line change
Expand Up		@@ -3018,6 +3018,52 @@ and type_expect_ ~context ?in_function ?(recarg = Rejected) env sexp ty_expected
		raise (Error (loc, env, Not_subtype (tr1, tr2, ctx))));
		(arg, ty', cty')
		in
		(* After a successful coercion, if we coerced between concrete variant
		types, mark the intersection of constructors as positively used for the
		target type to avoid spurious "constructor ... is never used" warnings
		when values are introduced via coercions. *)
		(try
		let _p_src, _p_src_conc, src_decl =
		Ctype.extract_concrete_typedecl env arg.exp_type
		in
		let p_tgt, p_tgt_conc, tgt_decl =
		Ctype.extract_concrete_typedecl env ty'
		in
		match (src_decl.type_kind, tgt_decl.type_kind) with
		\| Type_variant src_cons, Type_variant tgt_cons ->
		let module StringSet = Set.Make (String) in
		let src_set =
		List.fold_left
		(fun acc (c : Types.constructor_declaration) ->
		StringSet.add (Ident.name c.cd_id) acc)
		StringSet.empty src_cons
		in
		let has_src name = StringSet.mem name src_set in
		let tgt_ty_name_conc = Path.last p_tgt_conc in
		(* Mark usage for the concrete target decl (implementation view). *)
		List.iter
		(fun (c : Types.constructor_declaration) ->
		let cname = Ident.name c.cd_id in
		if has_src cname then
		Env.mark_constructor_used Env.Positive env tgt_ty_name_conc
		tgt_decl cname)
		tgt_cons;
		(* If the target type path differs from its concrete decl (e.g.,
		when a signature exposes a private or abstract alias), also mark
		usage on the exposed target declaration so scheduled warnings
		attached to that declaration are cleared. *)
		if not (Path.same p_tgt p_tgt_conc) then
		let exposed_ty_name = Path.last p_tgt in
		let exposed_decl = Env.find_type p_tgt env in
		List.iter
		(fun (c : Types.constructor_declaration) ->
		let cname = Ident.name c.cd_id in
		if has_src cname then
		Env.mark_constructor_used Env.Positive env exposed_ty_name
		exposed_decl cname)
		tgt_cons
Copy link Member zthSep 5, 2025 Choose a reason for hiding this comment The reason will be displayed to describe this comment to others.Learn more. @cristianoc what does this mean in practice? I don't get it. Copy link CollaboratorAuthor cristianocSep 5, 2025 Choose a reason for hiding this comment The reason will be displayed to describe this comment to others.Learn more. not sure Copy link CollaboratorAuthor cristianocSep 5, 2025 Choose a reason for hiding this comment The reason will be displayed to describe this comment to others.Learn more. @codex what's the answer here? chatgpt-codex-connector[bot] reacted with eyes emoji Copy link chatgpt-codex-connectorbotSep 5, 2025 Choose a reason for hiding this comment The reason will be displayed to describe this comment to others.Learn more. The code around line 3064 handles cases where the type path used in the code (`p_tgt`) differs from the path of its concrete declaration (`p_tgt_conc`). This occurs when a module signature exposes a variant type via a private or abstract alias. When such a coercion happens, the code marks the overlapping constructors as “used” on both the concrete declaration and the exposed alias so that “constructor … is never used” warnings are cleared for both locationscompiler/ml/typecore.mlL3051-L3064 View task → Copy link CollaboratorAuthor cristianocSep 5, 2025 Choose a reason for hiding this comment The reason will be displayed to describe this comment to others.Learn more. @codex explain more in detail the terms used in your response and the mechanism to mark a constructor never used. Don't assume familiarity with the representation of types and type environments, but introduce enough context to understand this change. chatgpt-codex-connector[bot] reacted with eyes emoji Copy link chatgpt-codex-connectorbotSep 5, 2025 Choose a reason for hiding this comment The reason will be displayed to describe this comment to others.Learn more. Summary Expanded the comments around coercion handling to define`p_tgt` (path used in source code) versus`p_tgt_conc` (resolved declaration path) and to explain how`Env.mark_constructor_used` records constructor usage on both the concrete type and any exposed alias so “constructor … is never used” warnings are clearedcompiler/ml/typecore.mlL3051-L3064 Testing ⚠️`make format`(fails: dune: command not found) ⚠️`make test`(fails: KeyboardInterrupt during ninja build) Notes `apt-get update` failed with 403 errors, so required dependencies (e.g.,`dune`) could not be installed View task → Copy link Member zthSep 5, 2025 Choose a reason for hiding this comment The reason will be displayed to describe this comment to others.Learn more. If we remove this special case code, does the fix still work? If so I think we should remove it, and then add it again in the future if we discover real issues it would fix.
		\| _ -> ()
		with Not_found -> ());
		rue
		{
		exp_desc = arg.exp_desc;
Expand Down
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,10 @@
		type a = A \| B

		// b spreads a and adds one more constructor
		type b =
		\| ...a
		\| C

		let upcast = (x: a): b => (x :> b)

		let _ = upcast(A)
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,46 @@
		found a compiler bug: https://github.com/rescript-lang/rescript/issues/7838 (due to the coercing between variants)
		GitHub
		Incorrect compiler warning (constructor is never used to build valu...
		https://rescript-lang.org/try?version=v11.1.4&module=esmodule&code=LYewJgrgNgpgBAUQG4wHYBc4F44G8BQcc6AngA7yY4A+iKGAgvoXLJgBYCGqYs2cAChj10ALmIBKbAD48LIgGcA7gEt0AY3ZxhaTASJFayXQxmCJ8uAF8WNm-l...
		Incorrect compiler warning (constructor is never used to build valu...
		[09:34]Gabriel Nordeborn: What is wrong here...?
		[09:35]jaap: The warning?
		[09:36]jaap: The variants are constructed in handle
		[09:37]jaap: I mean, the coercion constructs the variants
		[09:38]jaap: so if we have variantA :> variantB, we can mark that variantB is always constructed
		[09:38]Gabriel Nordeborn: Right, so if you adapt the example to not use coercion, it marks them as constructed?
		[09:39]Gabriel Nordeborn: It's good if you write this in the PR issue
		[09:39]Gabriel Nordeborn: Right now, just looking at the example, I did not understand what you're referring to
		[09:41]jaap: Yes
		[09:41]jaap: https://rescript-lang.org/try?version=v11.1.4&module=esmodule&code=LYewJgrgNgpgBAUQG4wHYBc4F44G8BQcc6AngA7yY4A+iKGAgvoXLJgBYCGqYs2cAChj10ALmIBKbAD48LIgGcA7gEt0AY3ZxhaTASJFayXQxmCJ8uAF8WNm-lCQ+AZRgAnFG+MYA4iHDi+qwwmGgAjhAwkeLe6AB0VLIQqGr4VvxBpBTE2Ja0cQWxCcxEbHBcPHw4nAokqOqCOhji6FJYskGKqhpaTXp5dCZmAhYG1rYsZeGRkfxCIjEiCW2yFbwwAspqmtoicoaDjGaxTERWEgC00ioA5qggbjBpzI7Q8K4e7rEAQpxggZMQtpUBEojBFroEmZkql0jhMuRKLkxvlCkt0CVghxuOt+DU6g15roWit9gYtj1droyQcTsNRgYbGdAaEQTN4Dgic1DvFWmY1rAuZhRLJWldbvdHs8gA
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		[09:41]jaap: here with two examples
		[09:41]jaap: also updated the ticket
		[09:44]cristianoc: The type system was created with no coercion in mind.
		[09:45]cristianoc: So fix warning —-> compiler error?
		[09:47]Gabriel Nordeborn: @jaap is this not a better example? https://rescript-lang.org/try?version=v11.1.4&module=esmodule&code=LYewJgrgNgpgBAUQG4wHYBc4F44G8BQcc6AngA7yY7JroCC+hcsmAFgIapizZwAUMFBgBcxAJTYAfHiZEAzgHcAlugDGrOINoyiRAD6Ih9KfzGy4AXyZWr+UJB4BlGACcULmhgDiIcKIJEpBTEvAYAdBGe6GHojEQsmqgAjhAwqaJU0hCoKvgWvAHE5JShcBFhUTFxzDBsnNzwOOxyJKiq-Foi4iaF8spqGp2YvXAGUXQmfGa6ltZMCWgpaY0dRhkSWNIcXLACRmIAtJJKAOaoIC4weYz20PDObq5RAELsYP7ztYlL6Ya0MSZsrl8jhCkESjhwpEjFVPnUditmq12ntaOseuZFCp1JojDpdGMjBNNqZzFYiOSaphFqlUrxUV1KugNlt6rsGZhhNJmYdjmcLlcrEA
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		[09:47]Gabriel Nordeborn: Using the switch fixes the warning of course because you're now constructing it explicitly
		[09:47]Gabriel Nordeborn: If you were to add the switch instead of coercion that'd work as well
		[09:48]Gabriel Nordeborn: But from what I understand the problem is that there's a value of a type that gets coerced to another type, and that other type is not marked as used
		[09:49]Gabriel Nordeborn: The value that's not coerced is marked, but the new type from the coercion is not marked
		[10:35]jaap: @cristianoc there is no way to fix this warning - you need to convert it to a switch expression, but that is a lot of redudant code potentially
		[10:36]cristianoc: That’s actually better. So the message is at least not inconsistent.
		[10:36]jaap: but the tricky part is that when coercing, the members that are in variantA need to marked as used, but not the new ones
		[10:36]cristianoc: I was worried: you remove the case, then get a compile error
		[10:37]jaap: Yes you do get an error - so it's not swallowing that
		[10:37]jaap: if you include it (warning) if you remove it (error)
		[10:38]cristianoc: There’s no such thing in the compiler. As ocaml does not have that. So it will be a genuine extension to handle this.
		And I don’t even know if possible.
		Is this global property? If so the. Compiler can’t do it.
		[10:38]jaap: but yeah seems to be a gap in the type system after adding the nice spreading of variants and coercing
		[10:39]jaap: No it's local - in the example you see the module signature added, so it only happens if you make the type private, and then coerce it
		[10:40]jaap: I guess there is some mechanism to mark members as constructed - and coercion is not doing that
		[10:40]cristianoc: Ok this is enough discussion.
		Paste it in a prompt and see if it gets close.
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		@@ -0,0 +1,13 @@

		[1;33mWarning number 37[0m
		[36m/.../fixtures/VariantCoercionWarnOnC.res[0m:[2m9:3-11:7[0m

		7 [2m│[0m let log: a => unit
		8 [2m│[0m } = {
		[1;33m9[0m [2m│[0m [1;33mtype b =[0m
		[1;33m10[0m [2m│[0m [1;33m \| ...a[0m
		[1;33m11[0m [2m│[0m [1;33m \| C[0m
		12 [2m│[0m
		13 [2m│[0m let log = (x: a) => Js.log((x :> b))

		unused constructor C.
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		@@ -0,0 +1,16 @@
		// Ensure unused-constructor warning still fires for new members introduced
		// only on the target side of a coercion.

		type a = A \| B

		module M: {
		let log: a => unit
		} = {
		type b =
		\| ...a
		\| C

		let log = (x: a) => Js.log((x :> b))
		}

		let _ = M.log(A)
Original file line number	Diff line number	Diff line change
Expand Up		@@ -62,7 +62,11 @@ let extractDocFromFile = async file => {
		}
		}

		let batchSize = OS.cpus()->Array.length
		// Some environments may report 0 CPUs; clamp to at least 1 to avoid zero-sized batches
		let batchSize = switch OS.cpus()->Array.length {
		\| n if n > 0 => n
		\| _ => 1
		}
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		let runtimePath = Path.join(["packages", "@rescript", "runtime"])

Expand Down
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,17 @@
		// Generated by ReScript, PLEASE EDIT WITH CARE


		function log(x) {
		console.log(x);
		}

		let DoNotWarn = {
		log: log
		};

		console.log("A");

		export {
		DoNotWarn,
		}
		/* Not a pure module */
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,15 @@
		// Repro for incorrect "constructor ... is never used" warning with coercions
		// This should compile cleanly without warnings when coercing from a -> b.

		type a = A \| B
		module DoNotWarn: {
		let log: a => unit
		} = {
		type b =
		\| ...a
		\| C

		let log = (x: a) => Js.log((x :> b))
		}

		let _ = DoNotWarn.log(A)