Description:
------------
Example 1:

<?php

class A{}
class B{}

trait X{
    abstract function test(B $b);
    function doSomething(){
        // can only be B. nothing should be allowed to change the signature of test(B)
        $this->test(new B);
    }
}

interface Contract{
    function test(A $a);
}

class Implementation implements Contract{
    use X; // incompatible, but compiles
    function test(A $a){}
}

-------------------------------------------------------

Example 2:

<?php

class A{}
class B{}

trait X{
    function tryA(A $a){
        $this->test($a);
    }
    function test(A $a){} // can't solve signatures incompatibility...
}

trait Y{
    function tryB(B $b){
        $this->test($b);
    }
    function test(B $b){} // ...when arguments are different
}


class Example{
    use X, Y{
        X::test insteadof Y; // this doesn't actually solve anything
    }
}

class A{}
class B{}

$x = new Example;
$x->tryA(new A);
$x->tryB(new B);

(insteadof can only work if resolution happens between two identical signatures)