fix #4513

When interface/implementation mismatches, reports the precise location

Author: bobzhang

lib/4.06.1/whole_compiler.ml

@@ -359557,12 +359557,31 @@ let report_error ppf errs =
   fprintf ppf "@[<v>%a%a@]" print_errs errs include_err err
 
 
+let better_candidate_loc (x : error list) = 
+    match x with 
+    | [ (_,_,Interface_mismatch _); (_,_,descr)]
+      ->
+        begin match descr with 
+        | Value_descriptions (_,d1,_) -> Some d1.val_loc
+        | Type_declarations (_,tdcl1,_,_) ->
+          Some tdcl1.type_loc
+        | Missing_field (_,loc,_) -> Some loc  
+        | _ -> None
+        end
+    | _ -> None    
+
 (* We could do a better job to split the individual error items
    as sub-messages of the main interface mismatch on the whole unit. *)
 let () =
   Location.register_error_of_exn
     (function
-      | Error err -> Some (Location.error_of_printer_file report_error err)
+      | Error err -> 
+        begin match better_candidate_loc err with 
+        | None -> 
+          Some (Location.error_of_printer_file report_error err)
+        | Some loc -> 
+          Some (Location.error_of_printer loc report_error err)  
+        end
       | _ -> None
     )