<===> input.scss
.a {
  display: block;
}

.b {
  @at-root (with: media) {
    @extend .a;
  }
}
<===> error
Error: Extend directives may only be used within rules.
input.scss
  Use --trace for backtrace.

<===> error-dart-sass
Error: @extend may only be used within style rules.
  ,
7 |     @extend .a;
  |     ^^^^^^^^^^
  '
  input.scss 7:5  root stylesheet