Fix is_rec_assign bug in simpleChecker.ml

Change to return `false` in the case of unknown type variables
because such type variables will turn out to be `Int`.
(`false` is returned in the case of `Int`.)

src/simpleChecker.ml

@@ -586,7 +586,7 @@ let is_rec_assign sub c t' =
       Hashtbl.find_opt sub.resolv v
       |> Option.map [%cast: typ]
       |> Option.map @@ check_loop h_rec
-      |> Option.value ~default:true
+      |> Option.value ~default:false
     | `Int -> false
     | `TyCons c ->
       let c_id = TyCons.unwrap c in