Async import - unbound module type

[CarlOlson]

Importing code from another workspace often creates an error:

  4 ┆ switch value {
  5 ┆ | Some(arr) =>
  6 ┆   module Array = await Belt.Array
  7 ┆   Js.log(arr->Array.keepMap(x => x))
  8 ┆ | None => ()

  Unbound module type __Belt_Array__

1. Here is the smallest example I've found: playgound

let fn = async value => {
  switch value {
  | Some(_) =>
    module Array = await Belt.Array
    ()
  | None => ()
  }
}

2. However, this does not error: playground

let fn = async value => {
  module Array = await Belt.Array
  switch value {
  | Some(_) => ()
  | None => ()
  }
}

3. Nor does this: playground

module type T = module type of Belt.Array

let fn = async value => {
  switch value {
  | Some(arr) =>
    module Array: T = await Belt.Array
    Js.log(arr->Array.keepMap(x => x))
  | None => ()
  }
}

But, (3) doesn't even produce an async import:

// Generated by ReScript, PLEASE EDIT WITH CARE
'use strict';

let Belt_Array = require("rescript/lib/js/Belt_Array.js");

async function fn(value) {
  if (value !== undefined) {
    console.log(Belt_Array.keepMap(value, x => x));
    return;
  }
  
}

exports.fn = fn;
/* No side effect */

This has been tested on 11.1.4 and 12.0.0-alpha.5, but the master branch has not been tested.

While importing Belt or Js asynchronously isn't desired, this error happens in my project in several useful locations.

[zth]

cc @mununki

[mediremi]

On v12.0.0-beta.6, annotating an async function with its return type triggers this bug:

// Unbound module type __Belt_Array__
let fn = async (): int => {
  module Array = await Belt.Array

  42
}

// No error
let fn: unit => promise<int> = async () => {
  module Array = await Belt.Array

  42
}

// No error
let fn = async () => {
  module Array = await Belt.Array

  42
}