cannot narrow empty

[jvasileff]

This function (which is suspect, but valid) doesn't compile:

Iterable<Element, Absent> emptyify<Element, Absent>
        (Iterable<Element, Absent> stream)
        given Absent satisfies Null {
    if (stream.empty) {
        assert (is Iterable<Element, Absent> empty); // error
        return empty;
    }
    else {
        return stream;
    }
}

$ ceylon compile
source/simple/run.ceylon:5: error: cannot find symbol
        assert (is Iterable<Element, Absent> empty);
                ^
  symbol:   variable empty
  location: class emptyify_
ceylon compile: There was 1 error