From 7427a92004c874780c1abb5ca2c946846c83fbb1 Mon Sep 17 00:00:00 2001
From: Christoph Knittel <ck@cca.io>
Date: Fri, 23 Oct 2026 07:19:51 +0000
Subject: [PATCH] Translate char to number in genType

Why:
    `char` is an alias of `int` since #8892, but genType's table of builtin
    types had no row for it and does not expand the abbreviation, so a
    `char` in a genType signature was emitted as the TypeScript type `char`,
    which does not exist.

Representation modified (genType's TypeScript type of char):
Example:
    @genType
    let char0 = (x: char) => x
Before (text): export const char0: (x:char) => char = CoreJS.char0 as any;
After (text): export const char0: (x:number) => number = CoreJS.char0 as any;

Signed-off-by: Christoph Knittel <ck@cca.io>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Xk6AeCYpCK4SZRtHYKHGoe
---
 changes/gentype-char-number.fix.md                           | 1 +
 compiler/gentype/translate_type_expr_from_types.ml           | 2 +-
 .../gentype_tests/typescript-react-example/src/Core.gen.tsx  | 2 ++
 tests/gentype_tests/typescript-react-example/src/Core.res    | 3 +++
 tests/gentype_tests/typescript-react-example/src/Core.res.js | 5 +++++
 5 files changed, 12 insertions(+), 1 deletion(-)
 create mode 100644 changes/gentype-char-number.fix.md

diff --git a/changes/gentype-char-number.fix.md b/changes/gentype-char-number.fix.md
new file mode 100644
index 00000000000..89c236a7fe7
--- /dev/null
+++ b/changes/gentype-char-number.fix.md
@@ -0,0 +1 @@
+- genType emits `char` as the TypeScript type `number` instead of `char`, which TypeScript does not know.
diff --git a/compiler/gentype/translate_type_expr_from_types.ml b/compiler/gentype/translate_type_expr_from_types.ml
index 1bf3110af1f..feae883c15f 100644
--- a/compiler/gentype/translate_type_expr_from_types.ml
+++ b/compiler/gentype/translate_type_expr_from_types.ml
@@ -67,7 +67,7 @@ let translate_constr ~config ~params_translation ~(path : Path.t) ~type_env =
   in
   match (path |> path_to_list |> List.rev, params_translation) with
   | ["bool"], [] -> {dependencies = []; type_ = boolean_t}
-  | ["int"], [] -> {dependencies = []; type_ = number_t}
+  | (["int"] | ["char"]), [] -> {dependencies = []; type_ = number_t}
   | ["float"], [] -> {dependencies = []; type_ = number_t}
   | (["string"] | ["Stdlib"; "String"; "t"]), [] ->
     {dependencies = []; type_ = string_t}
diff --git a/tests/gentype_tests/typescript-react-example/src/Core.gen.tsx b/tests/gentype_tests/typescript-react-example/src/Core.gen.tsx
index 113efbfacaa..fb657ed7c48 100644
--- a/tests/gentype_tests/typescript-react-example/src/Core.gen.tsx
+++ b/tests/gentype_tests/typescript-react-example/src/Core.gen.tsx
@@ -51,6 +51,8 @@ export const taggedTemplate1: (x:((strings:TemplateStringsArray, ...values:strin
 
 export const date1: (x:Date) => Date = CoreJS.date1 as any;
 
+export const char0: (x:number) => number = CoreJS.char0 as any;
+
 export const bigint0: (x:bigint) => bigint = CoreJS.bigint0 as any;
 
 export const stdlibBigInt: (x:bigint) => bigint = CoreJS.stdlibBigInt as any;
diff --git a/tests/gentype_tests/typescript-react-example/src/Core.res b/tests/gentype_tests/typescript-react-example/src/Core.res
index 7f48f22459e..e0492b814e9 100644
--- a/tests/gentype_tests/typescript-react-example/src/Core.res
+++ b/tests/gentype_tests/typescript-react-example/src/Core.res
@@ -34,6 +34,9 @@ let taggedTemplate1 = (x: TaggedTemplate.t<string, string>) => x
 @genType
 let date1 = (x: Date.t) => x
 
+@genType
+let char0 = (x: char) => x
+
 @genType
 let bigint0 = (x: bigint) => x
 
diff --git a/tests/gentype_tests/typescript-react-example/src/Core.res.js b/tests/gentype_tests/typescript-react-example/src/Core.res.js
index 67291b187b7..db15342d01e 100644
--- a/tests/gentype_tests/typescript-react-example/src/Core.res.js
+++ b/tests/gentype_tests/typescript-react-example/src/Core.res.js
@@ -50,6 +50,10 @@ function date1(x) {
   return x;
 }
 
+function char0(x) {
+  return x;
+}
+
 function bigint0(x) {
   return x;
 }
@@ -119,6 +123,7 @@ export {
   taggedTemplate0,
   taggedTemplate1,
   date1,
+  char0,
   bigint0,
   stdlibBigInt,
   regexp1,