How to extend Bootstrap 4 classes by breakpoint

I want to extend certain bootstrap classes with media breakpoints (similar like bootstrap does it with col-6, col-md-6).

So I tried this:

@media (max-width: 800px) {
  table.table-md-sm {
    @extend .table-sm;
  }
}

However that gives me the SASS compiler error «You may not @extend selectors across media queries».

So how can I do this with pure SASS?