syntax/jsx_v4: fix forwardRef arity regression; mark internal impl uncurried when forwardRef is used (#7738)

- Fix JSX v4 React.forwardRef arity regression seen in v12.
- When forwardRef is used, make the internal implementation uncurried with arity 2.
- Add regression test tests/tests/src/forwardRef_regress.res (JSX v4 enabled).

Author: cristianoc

compiler/syntax/src/jsx_v4.ml

@@ -733,6 +733,7 @@ let map_binding ~config ~empty_loc ~pstr_loc ~file_name ~rec_flag binding =
       | _ -> Pat.record (List.rev patterns_with_label) Open
     in
     let expression =
+      (* Shape internal implementation to match wrapper: uncurried when using forwardRef. *)
       Exp.fun_ ~arity:(Some 1) ~async:is_async Nolabel None
         (Pat.constraint_ record_pattern
            (Typ.constr ~loc:empty_loc
@@ -748,6 +749,10 @@ let map_binding ~config ~empty_loc ~pstr_loc ~file_name ~rec_flag binding =
                 | _ -> [Typ.any ()]))))
         expression
     in
+    let expression =
+      if has_forward_ref then expression |> Ast_uncurried.uncurried_fun ~arity:2
+      else expression
+    in
     let expression =
       (* Add new tupes (type a,b,c) to make's definition *)
       newtypes

tests/tests/src/forwardRef_regress.res

@@ -0,0 +1,8 @@
+@@config({flags: ["-bs-jsx", "4"]})
+
+@react.component
+let make = React.forwardRef((~className=?, ~children, _ref) => {
+  // Constrain type variables to avoid value restriction
+  let _: option<string> = className
+  children
+})