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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);
}

Built-in functions

Some built-in functions accept a narrower type than their native signature says, e.g. random_bytes() only accepts a positive-int:

<?php

function bad(int $length): string {
    return random_bytes($length); // ArgumentTypeCoercion
}

Declare the narrower type of the parameter with @psalm-param:

<?php

/** @psalm-param positive-int $length */
function good(int $length): string {
    return random_bytes($length); // no issue
}

Or check the value before the call, or assert its type with @psalm-var:

<?php

function checked(int $length): string {
    if ($length < 1) {
        throw new InvalidArgumentException('The length must be positive');
    }

    return random_bytes($length); // no issue
}

function asserted(int $length): string {
    /** @psalm-var positive-int $length */
    return random_bytes($length); // no issue
}