Extending self can lead to non-semanticly different output

[mgreter]

.foo[disabled] {
    @extend .foo;
    background: blue;
}
.bar[disabled] {
  foo {
    @extend .bar;
    background: blue;
  }
}
foo {
  .baz[disabled] {
    @extend .baz;
    background: blue;
  }
}

ruby sass:

.foo[disabled] { background: blue; }
.bar[disabled] foo { background: blue; }
foo [disabled].baz { background: blue; }

libsass with #2052

[disabled].foo { background: blue; }
.bar[disabled] foo { background: blue; }
foo [disabled].baz { background: blue; }

//CC @chriseppstein any idea why ruby sass differs in .bar[disabled] vs [disabled].baz?

[chriseppstein]

@mgreter I'm no expert in the extend code. But it seems like this is the reason:
https://github.com/sass/sass/blob/stable/lib/sass/selector/simple_sequence.rb#L193-L195

This is a non-semantic difference in a corner case where the @extend is pointless (it's extending itself). I wouldn't even bother to write a test for this. Just whistle and walk away.

[xzyfer]

Rescoping this to 3.4.1 since it shouldn't block 3.4.0.

[mgreter]

Input:

.bar.baz {
  @extend .bar;
  background: blue;
}

Expected:

.bar.baz {
  background: blue;
}

Current:

.baz.bar {
  background: blue;
}

Reason:

if modified_original || !replace || groups.empty?
  # First Law of Extend: the result of extending a selector should
  # (almost) always contain the base selector.
  # See https://github.com/nex3/sass/issues/324.
  original = Sequence.new([SimpleSequence.new(members, @subject, source_range)])
  original.add_sources! sources
  groups.unshift original
end
groups.uniq!

[xzyfer]

IMHO we shouldn't worry about non-semantically different output anymore. Dart Sass already produces semantically equivalent but different output. In sass-spec we can added libsass specific output with expected_output-libsass.css. I personally don't think implementing this sorting behaviour is worth the performance, and maintainer overhead.

Up to you @mgreter if you think this is worth investing in. I'm happy to close it.