Currently, when a compound selector is extended, Sass will only replace instances of the full extendee with the extender. For example:

.a {x: y}
.b {x: y}
.a.b {x: y}

.c {@extend .a.b}

produces

.a {x: y}
.b {x: y}
.a.b, .c {x: y}

when it should produce

.a, .c {x: y}
.b, .c {x: y}
.a.b, .c {x: y}

according to the semantics of @extend. We should fix this, especially as CSS moves closer to supporting @extend natively.

Tasks:

• Deprecate existing behavior in stable.
• Remove behavior from master.