extend does not work on a selector which has a variable (not extending a selector with a variable)

I understand from the docs that extends can't be used to extend a selector with a variable in it, but it is supposed to be able to be attached to an interpolated selector, according to the docs.

Using the exact code in the docs:

.bucket {
  color: blue;
}
@{variable}:extend(.bucket) {}
@variable: .selector;

I am getting the result

.bucket {
  color: blue;
}

not the expected (and documented) result

.bucket, .selector {
  color: blue;
}

I've verified this using the http://lesscss.org/less-preview/ playground, so it isn't something weird about my install or configuration.

Start by reproducing the documented interpolated-selector extend example in the Less preview playground and compare the actual output with the documented output. Trace the selector interpolation and extend handling, then confirm the fix produces the expected combined selector output.