Can't apply mixin as varible from a namespace

This might be related to https://github.com/less/less.js/issues/3405:

.mix (@n) {
  value: @n;
}

@var: .mix(var);

#lookup {
  @var: .mix(lookup);
  @bar: bar;
}

.example {
  @var();
  value2: #lookup[@bar];

  // This is what I'm trying to do, since the above works:
  #lookup[@var]();
}

Expected:

.example {
  value: lookup;
  value2: bar;
}

Actual: unexpected token.

In this case I can't find a workaround using detached rulesets, but since @var() works I expect the nested syntax #lookup[@var]() to work as well.

Start by reproducing the LESS snippet and comparing the generated CSS with the expected output. Trace how namespace lookup and variable mixin calls are parsed, then add coverage in the relevant existing test area. Done means #lookup@var compiles and produces the expected value declaration.