<===> input.scss
@debug(selector-extend(".a .b", "&b", ndll));

<===> error
Error: $extendee: Parent selectors aren't allowed here.
  ,
1 | &b
  | ^^
  '
  - 1:1  root stylesheet
  ,
1 | @debug(selector-extend(".a .b", "&b", ndll));
  |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  '
  input.scss 1:8  root stylesheet

<===> error-libsass
Error: Parent selectors aren't allowed here.
        on line 1:33 of sass-spec/spec/libsass-closed-issues/issue_2779/input.scss, in function `selector-extend`
        from line 1:7 of sass-spec/spec/libsass-closed-issues/issue_2779/input.scss
>> @debug(selector-extend(".a .b", "&b", ndll));

   --------------------------------^