Fix location issue for the treatment of async functions

Fixes https://github.com/rescript-lang/rescript-vscode/issues/698

Author: cristianoc

CHANGELOG.md

@@ -65,6 +65,7 @@ These are only breaking changes for unformatted code.
 - Fix issue in `Js.Promise2` where `then` and `catch` were returning `undefined` https://github.com/rescript-lang/rescript-compiler/pull/5997
 - Fix formatting of props spread for multiline JSX expression https://github.com/rescript-lang/rescript-compiler/pull/6006
 - Fix issue in the compiler back-end where async functions passed to an `@uncurry` external would be inlined and transformed in a way that loses async https://github.com/rescript-lang/rescript-compiler/pull/6010
+- Fix location issue for the treatment of `async` functions where hovering on the body with a type error would show `'a => promise<'a>` everywhere https://github.com/rescript-lang/rescript-compiler/pull/6012
 
 #### :nail_care: Polish
 

jscomp/frontend/ast_async.ml

@@ -1,6 +1,5 @@
-let add_promise_type ~async (result : Parsetree.expression) =
+let add_promise_type ?(loc=Location.none) ~async (result : Parsetree.expression) =
   if async then
-    let loc = result.pexp_loc in
     let unsafe_async =
       Ast_helper.Exp.ident ~loc
         { txt = Ldot (Ldot (Lident "Js", "Promise"), "unsafe_async"); loc }
@@ -18,16 +17,16 @@ let add_async_attribute ~async (body : Parsetree.expression) =
     }
   else body
 
-let rec add_promise_to_result (e : Parsetree.expression) =
+let rec add_promise_to_result ~loc (e : Parsetree.expression) =
   match e.pexp_desc with
   | Pexp_fun (label, eo, pat, body) ->
-      let body = add_promise_to_result body in
+      let body = add_promise_to_result ~loc body in
       { e with pexp_desc = Pexp_fun (label, eo, pat, body) }
-  | _ -> add_promise_type ~async:true e
+  | _ -> add_promise_type ~loc ~async:true e
 
 let make_function_async ~async (e : Parsetree.expression) =
   if async then
     match e.pexp_desc with
-    | Pexp_fun _ -> add_promise_to_result e
+    | Pexp_fun (_, _, {ppat_loc}, _) -> add_promise_to_result ~loc:ppat_loc e
     | _ -> assert false
   else e