diff --git a/src/ast_builder.ml b/src/ast_builder.ml
index b1642a641..2ea853753 100644
--- a/src/ast_builder.ml
+++ b/src/ast_builder.ml
@@ -33,16 +33,6 @@ module Default = struct
       ppat_desc = Ppat_construct (lid, Option.map p ~f:(fun p -> ([], p)));
     }
 
-  let pexp_fun ~loc (label : arg_label) expr p e =
-    let pparam_desc = Pparam_val (label, expr, p) in
-    let case = { pparam_desc; pparam_loc = loc } in
-    {
-      pexp_loc_stack = [];
-      pexp_attributes = [];
-      pexp_loc = loc;
-      pexp_desc = Pexp_function ([ case ], None, Pfunction_body e);
-    }
-
   let pexp_function_cases ~loc cases =
     {
       pexp_loc_stack = [];
@@ -51,7 +41,24 @@ module Default = struct
       pexp_desc = Pexp_function ([], None, Pfunction_cases (cases, loc, []));
     }
 
-  let pexp_function ~loc cases = pexp_function_cases ~loc cases
+  (* let pexp_function ~loc cases = pexp_function_cases ~loc cases *)
+
+  let add_fun_params return_constraint ~loc params body =
+    match params with
+    | [] -> body
+    | _ -> (
+        match body.pexp_desc with
+        | Pexp_function (more_params, constraint_, func_body) ->
+            pexp_function ~loc (params @ more_params) constraint_ func_body
+        | _ ->
+            assert (match params with [] -> false | _ -> true);
+            pexp_function ~loc params return_constraint (Pfunction_body body))
+
+  let pexp_fun ~loc (label : arg_label) expr p e =
+    let param : function_param =
+      { pparam_desc = Pparam_val (label, expr, p); pparam_loc = loc }
+    in
+    add_fun_params ~loc None [ param ] e
 
   let value_binding ~loc ~pat ~expr =
     value_binding ~loc ~pat ~expr ~constraint_:None
diff --git a/src/ast_builder.mli b/src/ast_builder.mli
index 42749f804..ea33f819d 100644
--- a/src/ast_builder.mli
+++ b/src/ast_builder.mli
@@ -35,13 +35,6 @@ module Default : sig
       (label loc list * pattern) option ->
       pattern
 
-    val pexp_function :
-      loc:location ->
-      function_param list ->
-      type_constraint option ->
-      function_body ->
-      expression
-
     val value_binding :
       ?constraint_:value_constraint ->
       loc:location ->
@@ -63,12 +56,12 @@ module Default : sig
   val ppat_construct :
     loc:location -> longident loc -> pattern option -> pattern
 
-  val pexp_function : loc:location -> Import.cases -> expression
-  [@@ocaml.deprecated "use pexp_function_cases instead."]
-  (** @deprecated
-        This function will be used to construct a {! Parsetree.Pexp_function }
-        in the next release, to retain its current functionality migrate to
-        {! pexp_function_cases}. *)
+  val pexp_function :
+    loc:location ->
+    function_param list ->
+    type_constraint option ->
+    function_body ->
+    expression
 
   val pexp_function_cases : loc:location -> Import.cases -> expression
   (** [pexp_function_cases] builds an expression in the shape
diff --git a/traverse/ppxlib_traverse.ml b/traverse/ppxlib_traverse.ml
index eaba6c1dc..48e21caf7 100644
--- a/traverse/ppxlib_traverse.ml
+++ b/traverse/ppxlib_traverse.ml
@@ -520,26 +520,11 @@ let gen_mapper ~(what : what) td =
         | None -> what#any ~loc
         | Some te -> type_expr_mapper ~what te)
   in
-  let params =
-    List.map
-      ~f:(fun (ty, _) ->
-        let loc = ty.ptyp_loc in
-        let desc =
-          match ty.ptyp_desc with
-          | Ptyp_var s -> pvar ~loc ("_" ^ s)
-          | _ -> ppat_any ~loc
-        in
-        let pparam_desc = Pparam_val (Nolabel, None, desc) in
-        { pparam_loc = loc; pparam_desc })
-      td.ptype_params
-  in
-  let pexp_desc = Pexp_function (params, None, Pfunction_body body) in
-  {
-    pexp_desc;
-    pexp_loc = td.ptype_loc;
-    pexp_loc_stack = [];
-    pexp_attributes = [];
-  }
+  List.fold_right td.ptype_params ~init:body ~f:(fun (ty, _) acc ->
+      let loc = ty.ptyp_loc in
+      match ty.ptyp_desc with
+      | Ptyp_var s -> pexp_fun ~loc Nolabel None (pvar ~loc ("_" ^ s)) acc
+      | _ -> pexp_fun ~loc Nolabel None (ppat_any ~loc) acc)
 
 let type_deps =
   let collect =