<===> _module.scss
.foo, & {
  background: red;
}

.foo, &:before {
  background: red;
}
<===> input.scss
@import "module";
<===> error
Error: Base-level rules cannot contain the parent-selector-referencing character '&'.
        on line 1 of _module.scss
        from line 1 of input.scss
  Use --trace for backtrace.

<===> error-libsass
Error: Top-level selectors may not contain the parent selector "&".
        on line 1:9 of input.scss
>> .foo, & {
   ------^

<===> error-dart-sass
Error: Top-level selectors may not contain the parent selector "&".
  ,
1 | .foo, & {
  | ^^^^^^^^
  '
  _module.scss 1:1  @import
  input.scss 1:9    root stylesheet