chriseppstein/sass — comment on commit f60bbd47fb7cd574db6c78932ca0fd523edd17df
File:  line 0
By nex3:

Chris and I talked about this offline, and ended up figuring out that we both agreed upon the first interpretation. The tests to the contrary in this commit are incorrect, and in fact do not print warnings at all.

There is still a case for which this implementation doesn't follow the first interpretation, though:

```scss
.bar {c: d}
.baz {e: f}
.foo {@extend .bar.baz}
```

I believe the way to go about making a more correct implementation is to keep track of two additional data structures in the cssize visitor:

* A set containing all `SimpleSelector`s used in rules in the stylesheet.
* A hash mapping extended `SimpleSelector`s to the `ExtendNode`s that extend them.

Then, before `do_extend`, just iterate through the hash and print an error if any of the keys aren't in the set.