In reply to MICRO_91:
Regardless of what the LRM says, defining sample() arguments with any other direction than input does not make any sense. A ref argument in a covergroup does not behave the same way as it does in a task/function. I think it was an oversight that the LRM does not restrict ref arguments to sample, or treat ref arguments the same as an input. See 0006817: Enable passing nets by const ref to functions and tasks - Accellera Mantis