Description:
------------
If a concrete method implemented by a trait is declared as returning "self", it is considered incompatible with an abstract declaration of that method in another trait.

Test script:
---------------
trait AbstractTrait {
    abstract public function selfReturner() : self;
}

trait ConcreteTrait {
    public function selfReturner() : self {
        return $this;
    }
}

class Test {
    use AbstractTrait;
    use ConcreteTrait;
}

Expected result:
----------------
The AbstractTrait trait should be satisfied by the implementation provided by ConcreteTrait.

Actual result:
--------------
Fatal error: Declaration of ConcreteTrait::selfReturner(): ConcreteTrait must be compatible with AbstractTrait::selfReturner(): AbstractTrait