Fix the issue by relying on the heuristic when at origin.

There might still be a problem when the comment is associated to the declaration.

src/analysis/locate.ml

@@ -933,7 +933,9 @@ let get_doc ~config ~env ~local_defs ~comments ~pos =
           { loc_start = pos; loc_end = pos; loc_ghost = true }
         in
         from_uid ~loc uid
-      | `At_origin | `Missing_labels_namespace -> `No_documentation
+      | `At_origin ->
+        `Found { loc_start = pos; loc_end = pos; loc_ghost = true }
+      | `Missing_labels_namespace -> `No_documentation
       | `Builtin _ -> `Builtin
       | (`Not_in_env _ | `Not_found _ |`File_not_found _ )
         as otherwise -> otherwise
@@ -966,7 +968,8 @@ let get_doc ~config ~env ~local_defs ~comments ~pos =
     let (_, deepest_before) =
       Mbrowse.(leaf_node @@ deepest_before loc.loc_start [browse])
     in
-    (* based on https://v2.ocaml.org/manual/doccomments.html#ss:label-comments: *)
+    (* based on:
+       https://v2.ocaml.org/manual/doccomments.html#ss:label-comments: *)
     let after_only = begin match deepest_before with
       | Browse_raw.Constructor_declaration _ -> true
       (* The remaining `true` cases are currently not reachable *)

tests/test-dirs/document/issue1540.t

@@ -4,10 +4,9 @@
   > let _ = id ()
   > EOF
 
-FIXME: Merlin should return the docstring
   $ $MERLIN single document -position 2:5 \
   > -filename doc.ml <doc.ml | jq '.value'
-  "No documentation available"
+  "whatever"
 
   $ $MERLIN single document -position 3:9 \
   > -filename doc.ml <doc.ml | jq '.value'