I tried re-creating this, and .someclass.notip was being generated for me but .someclass:not(.notip) was not, for as long as I did not have the @mixin tip() defined. Once I had that, it all worked.
http://sassmeister.com/gist/9775949
$dropdown-width: 100px;
$comp-tip: true;
@mixin tip($pos:right) {
}
@mixin dropdown-pos($pos:right) {
&:not(.notip) {
@if $comp-tip == true{
@if $pos == right {
top:$dropdown-width * -0.6;
background-color: #f00;
@include tip($pos:$pos);
}
}
}
&.notip {
@if $pos == right {
top: 0;
left:$dropdown-width * 0.8;
background-color: #00f;
}
}
}
.someclass { @include dropdown-pos(); }
EDIT: http://sassmeister.com/ is a good place to debug your SASS because it gives you error messages. Undefined mixin 'tip'. it what I get when I remove @mixin tip($pos:right) { }