Fix issue with async and newtype in uncurried mode.

Fixes #6587

Author: cristianoc

jscomp/ml/ast_async.ml

@@ -13,12 +13,18 @@ let add_promise_type ?(loc = Location.none) ~async
 
 let add_async_attribute ~async (body : Parsetree.expression) =
   if async then
-    {
-      body with
-      pexp_attributes =
-        ({txt = "res.async"; loc = Location.none}, PStr [])
-        :: body.pexp_attributes;
-    }
+   (   
+      match body.pexp_desc with
+      | Pexp_construct (x, Some e) when Ast_uncurried.exprIsUncurriedFun body ->
+        {body with pexp_desc = Pexp_construct (x, Some {e  with pexp_attributes =
+        ({txt = "res.async"; loc = Location.none}, PStr []) :: e.pexp_attributes} )}
+      | _ ->
+        {
+          body with
+          pexp_attributes =
+            ({txt = "res.async"; loc = Location.none}, PStr [])
+            :: body.pexp_attributes;
+        })
   else body
 
 let rec add_promise_to_result ~loc (e : Parsetree.expression) =

jscomp/test/async_await.js

@@ -16,3 +16,7 @@ let arr = [1, 2, 3]
 
 let toplevelAwait = await topFoo()
 let toplevelAwait2 = arr[await topFoo()]
+
+let f = async (type input, value: input) => {
+  await Js.Promise.resolve(. 1)
+}