sass-site/source/documentation/breaking-changes/extend-compound.md

---
title: 'Breaking Change: Extending Compound Selectors'
introduction: >
  LibSass currently allows compound selectors like `.message.info` to be
  [extended](/documentation/at-rules/extend), but the way it was extended doesn't match the
  way `@extend` is meant to work.
---

{% compatibility true, false, null, false %}{% endcompatibility %}

When one selector extends another, Sass styles all elements that match the
extender as though they also match the class being extended. In other words, if
you write `.heads-up {@extend .info}`, it works just like you replaced
`class="heads-up"` in your HTML with `class="heads-up info"`.

Following that logic, you'd expect that `.heads-up {@extend .message.info}` to
work like replacing `class="heads-up"` with `class="heads-up info message"`. But
that's not how it works right now in LibSass and Ruby Sass--instead of adding
`.heads-up` to every selector that has _either `.info` or `.message`_, it only
adds it to selectors that have _`.info.message` together_.

<!-- prettier-ignore-start -->
{% codeExample 1, false %}
// These should both be extended, but they won't be.
.message {
  border: 1px solid black;
}
.info {
  font-size: 1.5rem;
}

.heads-up {
  @extend .message.info;
}
===
// These should both be extended, but they won't be.
.message
  border: 1px solid black

.info
  font-size: 1.5rem


.heads-up
  @extend .message.info
{% endcodeExample %}
<!-- prettier-ignore-end -->

To fix this issue, avoid more confusion, and keep the implementation clean and
efficient the ability to extend compound selectors is unsupported in Dart Sass
and will be removed in a future version of LibSass. For compatibility, users
should extend each simple selector separately instead:

<!-- prettier-ignore-start -->
{% codeExample 2 %}
.message {
  border: 1px solid black;
}
.info {
  font-size: 1.5rem;
}

.heads-up {
  @extend .message, .info;
}
===
.message
  border: 1px solid black

.info
  font-size: 1.5rem


.heads-up
  @extend .message, .info
{% endcodeExample %}
<!-- prettier-ignore-end -->

{% headsUp %}
Because Sass doesn't know the details of the HTML the CSS is going to style,
any `@extend` might need to generate extra selectors that won't apply to your
HTML in particular. This is especially true when switching away from extending
compound selectors.

Most of the time, these extra selectors won't cause any problems, and will
only add a couple extra bytes to gzipped CSS. But some stylesheets might be
relying more heavily on the old behavior. In that case, we recommend replacing
the compound selector with a [placeholder selector][].

[placeholder selector]: /documentation/style-rules/placeholder-selectors

<!-- prettier-ignore-start -->
{% codeExample 3 %}
// Instead of just `.message.info`.
%message-info, .message.info {
  border: 1px solid black;
  font-size: 1.5rem;
}

.heads-up {
  // Instead of `.message.info`.
  @extend %message-info;
}
===
// Instead of just `.message.info`.
%message-info, .message.info
  border: 1px solid black
  font-size: 1.5rem


.heads-up
  // Instead of `.message.info`.
  @extend %message-info
{% endcodeExample %}
<!-- prettier-ignore-end -->

{% endheadsUp %}