Fix uncaught Unify exception in filter_arrow

ocaml/testsuite/tests/typing-layouts/basics_beta.ml

@@ -545,3 +545,26 @@ Line 3, characters 15-40:
                    ^^^^^^^^^^^^^^^^^^^^^^^^^
 Error: Type 'a has layout value, which is not a sublayout of immediate.
 |}]
+
+(****************************************************)
+(* Test 35: check bad layout error in filter_arrow *)
+
+module M35_1 : sig
+  val x : string
+end = struct
+  type ('a : immediate) t = 'a
+
+  let f : 'a t -> 'a = fun x y z -> x
+
+  let x = f (assert false)
+
+end;;
+
+[%%expect {|
+Line 6, characters 23-37:
+6 |   let f : 'a t -> 'a = fun x y z -> x
+                           ^^^^^^^^^^^^^^
+Error: This expression has type 'a -> 'b
+       but an expression was expected of type 'a -> 'b
+       'a -> 'b has layout value, which is not a sublayout of immediate.
+|}]

ocaml/typing/ctype.ml

@@ -3858,7 +3858,17 @@ let filter_arrow env t l ~force_tpoly =
     Tvar { layout } ->
       let t', arrow_desc = function_type (get_level t) in
       link_type t t';
-      constrain_type_layout_exn env Unify t' layout;
+
+      begin match constrain_type_layout env t' layout with
+      | Ok _ -> ()
+      | Error err ->
+        raise (Filter_arrow_failed
+                 (Unification_error
+                    (expand_to_unification_error
+                       env
+                       [Diff {got = t'; expected = t}; Bad_layout (t',err)])))
+      end;
+
       arrow_desc
   | Tarrow((l', arg_mode, ret_mode), ty_arg, ty_ret, _) ->
       if l = l' || !Clflags.classic && l = Nolabel && not (is_optional l')