vlib/v/checker/tests/mut_interface_param_err.vv:15:27: notice: unused parameter: `widget`
   13 | }
   14 | 
   15 | fn (mut w Window) add(mut widget Widget) {}
      |                           ~~~~~~
   16 | 
   17 | fn main() {
vlib/v/checker/tests/mut_interface_param_err.vv:20:10: error: method `add` parameter `widget` is `mut`, so use `mut btn` instead
   18 |     mut win := Window{}
   19 |     mut btn := Button{}
   20 |     win.add(btn) // should be an error here
      |             ~~~
   21 | }