Fix printing of user-defined option type used for optional arguments (#…

…3135)

ocaml/testsuite/tests/printing-types/user-option-args.ml

@@ -0,0 +1,10 @@
+(* TEST
+ toplevel;
+*)
+
+(** Test that type of optional argument is printed, even if using a non-predef
+    option type *)
+
+type nonrec 'a maybe = 'a option;;
+
+let create_property ?x () = let _ =  (x : int maybe) in ();;

ocaml/testsuite/tests/printing-types/user-option-args.ocaml.reference

@@ -0,0 +1,3 @@
+type nonrec 'a maybe = 'a option
+val create_property : ?x:int -> unit -> unit = <fun>
+

ocaml/typing/printtyp.ml

@@ -1435,7 +1435,9 @@ let rec tree_of_typexp mode alloc_mode ty =
         let arg_mode = Alloc.zap_to_legacy marg in
         let t1 =
           if is_optional l then
-            match get_desc (tpoly_get_mono ty1) with
+            match
+              get_desc (Ctype.expand_head !printing_env (tpoly_get_mono ty1))
+            with
             | Tconstr(path, [ty], _)
               when Path.same path Predef.path_option ->
                 tree_of_typexp mode arg_mode ty
@@ -2273,7 +2275,7 @@ let rec tree_of_class_type mode params =
       in
       let tr =
        if is_optional l then
-         match get_desc ty with
+         match get_desc (Ctype.expand_head !printing_env ty) with
          | Tconstr(path, [ty], _) when Path.same path Predef.path_option ->
              tree_of_typexp mode ty
          | _ -> Otyp_stuff "<hidden>"