Nested loop fails in 1.7.0

[SimonHarte]

The following code worked until 1.6.3 but now fails in 1.7.0 with a syntax error Cannot call method 'concat' of undefined:

.test() {
    .loop (@index) when (@index > 0) {
        // create css selector
        .@{index} {
            background: @index;
        }
        // next iteration
        .test > .loop(@index - 1);
    }
}
.test > .loop(12);

It is a simplified test version of this early example but nested in an unlocker mixin.

It seems to have a problem with the when () condition, if you take that and the next iteration away (to prevent infinite loop) it "works" but of course doesn't do a loop anymore.

[seven-phases-max]

See #2001. Fixed with #2002.