lexically_scoped_ampersand.scss

@mixin default-arg($s: &) {
  default-arg: inspect($s);
}

@mixin defaulted-in-body($s: null) {
  $s: & !default;
  defaulted-in-body: inspect($s);
}

output {
  @include default-arg;
  @include defaulted-in-body;
}

output.css

output {
  default-arg: null;
  defaulted-in-body: (output,); }