From 2022164e480d71d6365ded80412b7d78865582bf Mon Sep 17 00:00:00 2001
From: Christoph Knittel <ck@cca.io>
Date: Sun, 30 Nov 2025 10:56:39 +0100
Subject: [PATCH 1/2] Fix external signature inclusion for opaque function
 types

---
 compiler/ml/includecore.ml                   | 18 ++++++++++++++++++
 compiler/ml/primitive.ml                     |  3 +++
 compiler/ml/primitive.mli                    |  3 +++
 tests/tests/src/gpr_8038_opaque_external.mjs |  9 +++++++++
 tests/tests/src/gpr_8038_opaque_external.res | 10 ++++++++++
 5 files changed, 43 insertions(+)
 create mode 100644 tests/tests/src/gpr_8038_opaque_external.mjs
 create mode 100644 tests/tests/src/gpr_8038_opaque_external.res

diff --git a/compiler/ml/includecore.ml b/compiler/ml/includecore.ml
index b1db9e5f16..05753d36c8 100644
--- a/compiler/ml/includecore.ml
+++ b/compiler/ml/includecore.ml
@@ -24,6 +24,22 @@ open Typedtree
 
 exception Dont_match
 
+(* When comparing externals in signatures, re-derive arity/from_constructor from
+   the value's type so abstract aliases (e.g. opaque function types) don't keep
+   default zeros stored in the primitive descriptor, which would make equal
+   externals look different. *)
+let normalize_primitive ~env val_type (prim : Primitive.description) =
+  match Ctype.get_arity env val_type with
+  | Some prim_arity ->
+    let prim_from_constructor =
+      match (Ctype.repr val_type).desc with
+      | Tconstr _ -> true
+      | _ -> prim.prim_from_constructor
+    in
+    Primitive.with_arity prim ~arity:prim_arity
+      ~from_constructor:prim_from_constructor
+  | None -> prim
+
 let value_descriptions ~loc env name (vd1 : Types.value_description)
     (vd2 : Types.value_description) =
   Builtin_attributes.check_deprecated_inclusion ~def:vd1.val_loc
@@ -31,6 +47,8 @@ let value_descriptions ~loc env name (vd1 : Types.value_description)
   if Ctype.moregeneral env true vd1.val_type vd2.val_type then
     match (vd1.val_kind, vd2.val_kind) with
     | Val_prim p1, Val_prim p2 ->
+      let p1 = normalize_primitive ~env vd1.val_type p1 in
+      let p2 = normalize_primitive ~env vd2.val_type p2 in
       if !Primitive.coerce p1 p2 then Tcoerce_none else raise Dont_match
     | Val_prim p, _ ->
       let pc =
diff --git a/compiler/ml/primitive.ml b/compiler/ml/primitive.ml
index 3ddc60a488..f632606e9a 100644
--- a/compiler/ml/primitive.ml
+++ b/compiler/ml/primitive.ml
@@ -30,6 +30,9 @@ type description = {
 
 let set_transformed_jsx d ~transformed_jsx = {d with transformed_jsx}
 
+let with_arity d ~arity ~from_constructor =
+  {d with prim_arity = arity; prim_from_constructor = from_constructor}
+
 let coerce : (description -> description -> bool) ref =
   ref (fun (p1 : description) (p2 : description) -> p1 = p2)
 
diff --git a/compiler/ml/primitive.mli b/compiler/ml/primitive.mli
index 8f5c58100d..c5b43c3b30 100644
--- a/compiler/ml/primitive.mli
+++ b/compiler/ml/primitive.mli
@@ -27,6 +27,9 @@ type description = private {
 
 val set_transformed_jsx : description -> transformed_jsx:bool -> description
 
+val with_arity :
+  description -> arity:int -> from_constructor:bool -> description
+
 (* Invariant [List.length d.prim_native_repr_args = d.prim_arity] *)
 
 val parse_declaration :
diff --git a/tests/tests/src/gpr_8038_opaque_external.mjs b/tests/tests/src/gpr_8038_opaque_external.mjs
new file mode 100644
index 0000000000..25597c4d0a
--- /dev/null
+++ b/tests/tests/src/gpr_8038_opaque_external.mjs
@@ -0,0 +1,9 @@
+// Generated by ReScript, PLEASE EDIT WITH CARE
+
+
+let B = {};
+
+export {
+  B,
+}
+/* No side effect */
diff --git a/tests/tests/src/gpr_8038_opaque_external.res b/tests/tests/src/gpr_8038_opaque_external.res
new file mode 100644
index 0000000000..fca5c3134c
--- /dev/null
+++ b/tests/tests/src/gpr_8038_opaque_external.res
@@ -0,0 +1,10 @@
+/* Regression for https://github.com/rescript-lang/rescript/issues/8038 */
+module type A = {
+  type t
+  @module external dep: t = "dep"
+}
+
+module B: A = {
+  type t = string => string
+  @module external dep: t = "dep"
+}

From 3a78abe66be8668c12c6503800204f8ad7087593 Mon Sep 17 00:00:00 2001
From: Christoph Knittel <ck@cca.io>
Date: Sun, 30 Nov 2025 11:16:13 +0100
Subject: [PATCH 2/2] CHANGELOG

---
 CHANGELOG.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2c1dbb4f74..388fb08246 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -21,6 +21,7 @@
 #### :bug: Bug fix
 
 - Rewatch: warnings for unsupported/unknown rescript.json fields. https://github.com/rescript-lang/rescript/pull/8031
+- Fix external signature inclusion for opaque function types. https://github.com/rescript-lang/rescript/pull/8045
 
 #### :memo: Documentation