Less extend from imported file not working vs mixin from imported file

Question

With the following two files a.less and b.less:

a.less:

.red {
    background: red;
}

and b.less:

@import "a.less"

.testA:extend(.red) {
}

.testB {
    .red;
}

The result for b.css is:

.testA {}
.testB {
    background: red;
}

Why does extend not work with the imported mixin/class definition for .testA? Is there a limitation on the extend syntax that requires the extended definition to be present in the same file, i.e. is this expected behavior?

Note that .testA gets compiled correctly if I move the .red mixin to b.less.

Edit: I am using the less binary precompiler, version 1.6.1

Edit: To be absolutely sure about typos and as I don't think there's a online service that allows several less files and imports, download the example source here - as a matter of fact .testa{} won't get rendered at all, but b.css naturally has a .red class defined.

Edit: When I compile from bash the extend works correctly, but using the less.app GUI or compiling via my IDE (with the same command as from bash) or via a local file watcher the extend does not get included correctly - could this somehow have to do with where the binary gets called from?

Answer 1

As @seven-phases-max pointed out, the problem is a differing version of the less preprocessor shipped in the GUI tool, which does not have the :extend feature.