How do I reference the parent selector at the end? [duplicate]

Answer 1

If you read the complete error, it should say this:

"&" may only be used at the beginning of a compound selector.

However, .dropdown-toggle.btn-primary is the same as .btn-primary.dropdown-toggle. So place the parent selector at the beginning:

.btn-primary {
  .open &.dropdown-toggle { color: blue; }
}

As of Sass 3.4, you can do that like this:

.btn-primary {
  .open .dropdown-toggle#{&} { color: blue; }
}