Breaking Change: Extending Compound Selectors

LibSass currently allows compound selectors like .message.info to be extended, but the way it was extended doesn't match the way @extend is meant to work.

Compatibility:
Dart Sass
✓
LibSass
✗
Ruby Sass
✗

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.

SCSS Syntax

// 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;
}

Sass Syntax

// 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

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:

SCSS Syntax

.message {
  border: 1px solid black;
}
.info {
  font-size: 1.5rem;
}

.heads-up {
  @extend .message, .info;
}

Sass Syntax

.message
  border: 1px solid black

.info
  font-size: 1.5rem


.heads-up
  @extend .message, .info

CSS Output

.message, .heads-up {
  border: 1px solid black;
}

.info, .heads-up {
  font-size: 1.5rem;
}

⚠️ Heads up!

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.

SCSS Syntax

// 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;
}

Sass Syntax

// 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

CSS Output

.heads-up, .message.info {
  border: 1px solid black;
  font-size: 1.5rem;
}