Need help understanding selector-extend() behavior with placeholder selectors.

[and...@skybound.ca]

I've been trying to understand the algorithm for selector-extend() and have a question.

Input:

a {
   foo: selector-extend('a.foo.baz', '.foo', '.baz');
}

Output:

a {
   foo: a.foo.baz, a.baz;
}

This makes sense so far.  For the second output selector, .foo was replaced with .baz, and was then removed because it is a duplicate class name.  However, when a placeholder selector appears in the first argument, the situation changes:

Input:

a {
   foo: selector-extend('%a.foo.baz', '.foo', '.baz');
}

Output:

a {
   foo: %a.baz;
}

Now the original selector is not being included in the output.  Indeed "%a.foo.baz" is a superselector of "%a.baz" and it makes sense to remove it.  But could someone please explain why this unification only happened when a placeholder selector appeared in the input, and not otherwise?

[Natalie Weizenbaum]

Sass guarantees that the selector resulting from an @extend will always match the original selector with the same specificity as that selector. This means that the original will never be omitted, even if a superselector of it is generated—except when it contains a placeholder. Because placeholders can't be emitted to the document anyway, specificity isn't a reason to keep them around. So they are omitted if a superselector is generated.

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