Fix partial application generalization for ...

CHANGELOG.md

@@ -20,6 +20,8 @@
 
 #### :bug: Bug fix
 
+- Fix partial application generalization for `...`. https://github.com/rescript-lang/rescript/pull/8343
+
 #### :memo: Documentation
 
 #### :nail_care: Polish

compiler/ml/typecore.ml

@@ -1803,6 +1803,11 @@ let rec is_nonexpansive exp =
     List.for_all (fun vb -> is_nonexpansive vb.vb_expr) pat_exp_list
     && is_nonexpansive body
   | Texp_function _ -> true
+  | Texp_apply {partial = true; _} ->
+    (* ReScript partial applications (`foo(args, ...)`) lower to wrapper
+       functions in codegen, so creating the partial itself is nonexpansive
+       like an explicit lambda. *)
+    true
   | Texp_apply {funct = e; args = (_, None) :: el} ->
     is_nonexpansive e && List.for_all is_nonexpansive_opt (List.map snd el)
   | Texp_match (e, cases, [], _) ->

tests/tests/src/PartialApplicationNoRuntimeCurry.mjs

@@ -18,9 +18,18 @@ function add5(extra) {
   return 5 + extra | 0;
 }
 
+function addHookPartial(extra) {
+  addHook(hook, extra);
+}
+
+addHook(hook, _x => {});
+
+addHook(hook, _x => {});
+
 export {
   f,
   add$1 as add,
   add5,
+  addHookPartial,
 }
-/* No side effect */
+/*  Not a pure module */

tests/tests/src/PartialApplicationNoRuntimeCurry.res

@@ -9,3 +9,11 @@ let f = u => {
 type fn2 = (int, int) => int
 let add: fn2 = (a, b) => a + b
 let add5: int => int = add(5, ...)
+
+type hook
+external hook: hook = "hook"
+external addHook: (hook, 'a => unit) => unit = "addHook"
+
+let addHookPartial = addHook(hook, ...)
+let _ = addHookPartial((_x: int) => ())
+let _ = addHookPartial((_x: string) => ())