Does this actually fix anything? I was trying to understand what's the actual issue here and I don't think changing it to uncurried makes the problem go away:

external useState :
  ((unit -> 'state)[@mel.uncurry]) ->
  'state * ((('state -> 'state)[@u]) -> unit) = "useState"
[@@mel.module "react"]

let x, setX = useState (fun _ -> 22)
let () = setX (fun [@u] _ -> 23)

$ melc -ppx melppx x.ml
// Generated by Melange
'use strict';

const Curry = require("melange.js/curry.js");
const React = require("react");

const match = React.useState(function () {
      return 22;
    });

const setX = match[1];

Curry._1(setX, (function (param) {
        return 23;
      }));

const x = match[0];

exports.x = x;
exports.setX = setX;
/* match Not a pure module */

BTW, my current understanding of why Melange inserts the Curry._1(...) call is because 'state -> 'state is a pretty interesting type for setState ('state could itself be a function!) -- it'd be nice to be able to tell melange to stop after the first application (instead of using Curry._1).

OK, I understand the issue now. it's rather like React.Uncurried.

This is fixed upstream in facebook/react next version (probably 19.0.1?) for now install the @next as seen here: 0bc3400

This PR was to ilustrate the problem, can be closed now.