Description:
------------
When generators were implemented, a generator function could return another generator even if it was a return by reference generator.

In PHP7 this now produces a warning because it is not a variable.

See http://3v4l.org/sFR11#vphp7@20141101

Test script:
---------------
<?php

function & genRefInner() {
    $var = 1;
    yield $var;
}

function & genRefOuter() {
    return genRefInner();
}

foreach(genRefOuter() as $i) {
    var_dump($i);
}

?>

Expected result:
----------------
int(1)

Actual result:
--------------
Notice: Only variable references should be returned by reference in /in/sFR11 on line 9
int(1)