Fix prefix parsing for method call

When completing module paths or record fields Merlin expects the
    beginning of the path and the `.` to be part of the prefix. But when
    accessing an object's methods, the prefix should not contain the `#`
    sign. We use a sub-matching group to that effect.

    - Prefix for [my_record.|] is ["my_record."] (handled by [name_or_label])
    - Prefix for [my_object#|] is [""] (handled by [method_call])

ocaml-lsp-server/src/prefix_parser.ml

@@ -24,6 +24,15 @@ include struct
          ])
   ;;
 
+  (**  When completing module paths or record fields Merlin expects the
+       beginning of the path and the `.` to be part of the prefix. But when
+       accessing an object's methods, the prefix should not contain the `#`
+       sign. We use a sub-matching group to that effect.
+
+       - Prefix for [my_record.|] is ["my_record."] (handled by [name_or_label])
+       - Prefix for [my_object#|] is [""] (handled by [method_call]) *)
+  let method_call = compile (seq [ char '#'; Re.group (rep name_char); stop ])
+
   (** matches let%lwt and let* style expressions. See
       here:https://v2.ocaml.org/manual/bindingops.html *)
   let monadic_bind =
@@ -42,8 +51,11 @@ let parse ~pos ~len text =
   (*Attempt to match each of our possible prefix types, the order is important
     because there is some overlap between the regexs*)
   let matched =
-    List.find_map [ name_or_label; monadic_bind; infix_operator ] ~f:(fun regex ->
-      Re.exec_opt ~pos ~len regex text)
+    List.find_map
+      [ name_or_label; method_call; monadic_bind; infix_operator ]
+      ~f:(fun regex -> Re.exec_opt ~pos ~len regex text)
   in
-  matched |> Option.map ~f:(fun x -> Re.Group.get x 0)
+  matched
+  |> Option.map ~f:(fun x ->
+    if Re.Group.test x 1 then Re.Group.get x 1 else Re.Group.get x 0)
 ;;

ocaml-lsp-server/test/e2e-new/completion.ml

@@ -1251,5 +1251,51 @@ let%expect_test "completion for object methods" =
   print_completions ~limit:3 source position;
   [%expect
     {|
-    No completions |}]
+    Completions:
+    {
+      "kind": 14,
+      "label": "in",
+      "textEdit": {
+        "newText": "in",
+        "range": {
+          "end": { "character": 34, "line": 0 },
+          "start": { "character": 34, "line": 0 }
+        }
+      }
+    }
+    {
+      "detail": "'a",
+      "kind": 2,
+      "label": "a_method",
+      "sortText": "0000",
+      "textEdit": {
+        "newText": "a_method",
+        "range": {
+          "end": { "character": 34, "line": 0 },
+          "start": { "character": 34, "line": 0 }
+        }
+      }
+    } |}]
+;;
+
+let%expect_test "completion for object methods" =
+  let source = {ocaml|let f (x : < a_method : 'a; ab_m : 'b >) = x#ab|ocaml} in
+  let position = Position.create ~line:0 ~character:49 in
+  print_completions ~limit:3 source position;
+  [%expect
+    {|
+    Completions:
+    {
+      "detail": "'b",
+      "kind": 2,
+      "label": "ab_m",
+      "sortText": "0000",
+      "textEdit": {
+        "newText": "ab_m",
+        "range": {
+          "end": { "character": 49, "line": 0 },
+          "start": { "character": 47, "line": 0 }
+        }
+      }
+    } |}]
 ;;