--- src/ppx/instrument.ml	2026-05-06 22:10:41
+++ src/ppx/instrument.ml	2026-05-06 22:11:05
@@ -1314,38 +1314,55 @@
             instrument_expr ~use_loc_of:e ~post:true (Exp.assert_ e_new)
 
           (* Expressions that have subexpressions that might not get visited. *)
-          | Pexp_function cases ->
-            traverse_cases ~is_in_tail_position:true cases
-            >>| fun cases_new ->
-            let cases, _, _, need_binding = instrument_cases cases_new in
-            if need_binding then
-              Exp.fun_ ~loc ~attrs
-                Ppxlib.Nolabel None ([%pat? ___bisect_matched_value___])
-                (Exp.match_ ~loc
-                  ([%expr ___bisect_matched_value___]) cases)
-            else
-              Exp.function_ ~loc ~attrs cases
-
-          | Pexp_fun (label, default_value, p, e) ->
-            begin match default_value with
-            | None ->
-              return None
-            | Some e ->
-              traverse ~is_in_tail_position:false e
-              >>| fun e ->
-              Some (instrument_expr e)
+          | Pexp_function (params, constraint_, body) ->
+            begin match body with
+            | Pfunction_cases (cases, cases_loc, cases_attrs) ->
+              traverse_cases ~is_in_tail_position:true cases
+              >>| fun cases_new ->
+              let cases, _, _, need_binding = instrument_cases cases_new in
+              if need_binding then
+                if params = [] then
+                  Exp.fun_ ~loc ~attrs
+                    Ppxlib.Nolabel None ([%pat? ___bisect_matched_value___])
+                    (Exp.match_ ~loc
+                      ([%expr ___bisect_matched_value___]) cases)
+                else
+                  {e with pexp_desc =
+                    Pexp_function (params, constraint_,
+                      Pfunction_body (
+                        Exp.fun_ ~loc Ppxlib.Nolabel None
+                          [%pat? ___bisect_matched_value___]
+                          (Exp.match_ ~loc
+                            ([%expr ___bisect_matched_value___]) cases)))}
+              else
+                {e with pexp_desc =
+                  Pexp_function (params, constraint_,
+                    Pfunction_cases (cases, cases_loc, cases_attrs))}
+            | Pfunction_body body_expr ->
+              let instrument_param param =
+                match param.Parsetree.pparam_desc with
+                | Pparam_val (label, Some dv, p) ->
+                  traverse ~is_in_tail_position:false dv
+                  >>| fun dv_new ->
+                  {param with Parsetree.pparam_desc =
+                    Pparam_val (label, Some (instrument_expr dv_new), p)}
+                | _ ->
+                  return param
+              in
+              collect_errors (List.map instrument_param params)
+              >>= fun new_params ->
+              traverse ~is_in_tail_position:true body_expr
+              >>| fun body_new ->
+              let body_new =
+                match body_new.pexp_desc with
+                | Pexp_function _ -> body_new
+                | Pexp_constraint (e', t) ->
+                  {body_new with pexp_desc = Pexp_constraint (instrument_expr e', t)}
+                | _ -> instrument_expr body_new
+              in
+              {e with pexp_desc =
+                Pexp_function (new_params, constraint_, Pfunction_body body_new)}
             end
-            >>= fun default_value ->
-            traverse ~is_in_tail_position:true e
-            >>| fun e ->
-            let e =
-              match e.pexp_desc with
-              | Pexp_function _ | Pexp_fun _ -> e
-              | Pexp_constraint (e', t) ->
-                {e with pexp_desc = Pexp_constraint (instrument_expr e', t)}
-              | _ -> instrument_expr e
-            in
-            Exp.fun_ ~loc ~attrs label default_value p e
 
           | Pexp_match (e, cases) ->
             traverse_cases ~is_in_tail_position cases
@@ -1418,7 +1435,7 @@
           | Pexp_lazy e ->
             let rec is_trivial_syntactic_value e =
               match e.Parsetree.pexp_desc with
-              | Pexp_function _ | Pexp_fun _ | Pexp_poly _ | Pexp_ident _
+              | Pexp_function _ | Pexp_poly _ | Pexp_ident _
               | Pexp_constant _ | Pexp_construct (_, None) ->
                 true
               | Pexp_constraint (e, _) | Pexp_coerce (e, _, _) ->
@@ -1446,7 +1463,7 @@
             >>| fun e ->
             let e =
               match e.pexp_desc with
-              | Pexp_function _ | Pexp_fun _ -> e
+              | Pexp_function _ -> e
               | _ -> instrument_expr e
             in
             Exp.poly ~loc ~attrs e t