Documentation error. Please fix it.

On this page: https://lesscss.org/features/#extend-feature-extend-all

This example does not work:

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

compiles to:

.bucket, .selector { color: blue; }

You need to write it like this:

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

compiles to:

.bucket, .selector { color: blue; }

As I understand it, LESS treats interpolation as "late binding", so :extend() cannot be applied to a selector that has not yet been formed at the parsing stage.

Open the linked Extend all documentation page and locate the example around the reported anchor. Compare the shown interpolation and :extend() forms, then update the documentation example and verify that the rendered page and documented compilation output are correct.