Simplify (and fix) the code adding variables of a function interface to delta...

Simplify (and fix) the code adding variables of a function interface to delta during the typing of its body.

typing/typer.ml

@@ -977,21 +977,13 @@ and type_check' loc env ed constr precise = match ed with
     in
       (* update \delta with all variables in t1 -> t2 *)
 
-    let deltaintf =
-      let union (t1,t2) =
-        Var.Set.union
-          (Types.all_vars(t1))
-          (Types.all_vars(t2))
-      in
-      match a.fun_iface with
-      |[] -> Var.Set.empty
-      |head::tail ->
-        List.fold_left (fun acc intf ->
-          Var.Set.inter (union intf) acc
-        ) (union head) tail
+    let delta_intf =
+      List.fold_left (fun acc (t1, t2) ->
+        Var.Set.(union acc (union (Types.all_vars t1) (Types.all_vars t2)))
+      ) env.delta a.fun_iface
     in
 
-    let env = {env with delta = Var.Set.union env.delta deltaintf } in
+    let env = {env with delta = delta_intf } in
 
       (* I check the body with all possible t1 -> t2 types *)
     let sll =