I believe that you cannot distribute multiple selectors this way.
Try using this:
@extend .btn; @extend .btn-link;
Although this seems a bit repetitive, it works fine in my codes.
EDIT: while reading through SASS_REFERENCE, I found that:
Multiple extensions can also be written using a comma-separated list of selectors. For example, @ extend.error, .attention is the same as @ extend.error; @ Extend.attention.
I found on the change list that this format was first introduced in version 3.1.15, so I suppose you are using an older version of Sass than this.
I highly recommend that you upgrade to the latest version, as it has a lot of great features, just make sure your codes are not broken by the update, although most inconsistencies can be done quite easily.
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