Extend syntax inconsistency (was "ParseError: Unrecognised input" on extends)

If I lint this file by lessc -l file:

.something {
    color: red;
}
body {
    nav {/* a lot more lines */
        ul {
            color: blue;
            /*v1*/
            li:extend(.something);
            /*v2
            li {
                &:extend(.something);
            }*/
        }
        /*v3
        ul li:extend(.something); */
    }
}

I receive a message that isn't even pointing me in the direction of the error:

ParseError: Unrecognised input in file on line 5, column 2:
4 body {
5       [red] nav { /* a lot more lines */ [/red]
6               ul {

There are two issues:

• The parser accepts only v2 of the the three expressions that appear equivalent to me. My understanding of less is limited so I'm expecting to have overlooked something. I'd appreciate some feedback.
• In this particular case the ParseError message is not helpful. Guessing from the rarity of misleading messages I've seen so far I'd blame extend. But I should investigate this further.

Start by reproducing the example with lessc -l file and compare the parser's handling of the v1, v2, and v3 extend forms. Trace the resulting parse error to determine why it reports the nav line; done means the accepted syntax and any corrected error location are covered by tests.