<===> input.scss
@function foo($value, $default: 13, $args...) {
  $res: $value + $default;
  @if length($args) != 0 {
    $res: $res + nth($args, 1);
  }
  @return $res;
}

.test {
  value: foo(3); // expected: 16
  value: foo(3, 4); // expected: 7
  value: foo(3, 4, 5, 6); // expected: 12
}

<===> output.css
.test {
  value: 16;
  value: 7;
  value: 12;
}