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ArgumentTypeCoercion

Emitted when calling a function with an argument which has a less specific type than the function expects

<?php

class A {}
class B extends A {}

function takesA(A $a) : void {
    takesB($a);
}
function takesB(B $b) : void {}

How to fix

You could add a typecheck before the call to takesB:

<?php

function takesA(A $a) : void {
    if ($a instanceof B) {
        takesB($a);
    }
}

Or, if you have control over the function signature of takesA you can change it to expect B:

<?php

function takesA(B $a) : void {
    takesB($a);
}