<===> input.scss
// `@content()` and `@content` should behave identically, as should
// `@include foo` and `@include foo using ()`.

no-parens {
  @mixin mixin {
    @content;
  }

  empty-using {
    @include mixin using () {
      x: y;
    }
  }

  defaults {
    @include mixin using ($arg1: value1, $arg2: value2) {
      arg1: $arg1;
      arg2: $arg2;
    }
  }
}

parens {
  @mixin mixin {
    @content();
  }

  no-using {
    @include mixin {
      x: y;
    }
  }

  empty-using {
    @include mixin using () {
      x: y;
    }
  }

  defaults {
    @include mixin using ($arg1: value1, $arg2: value2) {
      arg1: $arg1;
      arg2: $arg2;
    }
  }
}

<===> output.css
no-parens empty-using {
  x: y;
}
no-parens defaults {
  arg1: value1;
  arg2: value2;
}

parens no-using {
  x: y;
}
parens empty-using {
  x: y;
}
parens defaults {
  arg1: value1;
  arg2: value2;
}