<===> input.scss
@mixin foo($x, $y) { }

a {
  @include foo(1 2 3...);
}

<===> error-dart-sass
Error: Only 2 arguments allowed, but 3 were passed.
    ,
1   | @mixin foo($x, $y) { }
    |        =========== declaration
... |
4   |   @include foo(1 2 3...);
    |   ^^^^^^^^^^^^^^^^^^^^^^ invocation
    '
  input.scss 4:3  foo()
  input.scss 4:3  root stylesheet

<===> output.css
<===> warning
WARNING: Mixin foo takes 2 arguments but 3 were passed.
        on line 4 of input.scss
This will be an error in future versions of Sass.