Bug report

I want to have a method that trim an argument if it's a string, or return it as is if not.
An error Function hello() should return T but returns string. is reported.
If (on hello2), I tweak the return to tell it return string if the input is string, the error is not reported, but another one is reported if I call it
Return type of call to function hello2 contains unresolvable type.

<?php declare(strict_types = 1);

/** @template T of mixed
 * @param T $a
 * @return T
 */
function hello(mixed $a): mixed
{
    if (!is_string($a)) {
        return $a;
    }

	hello2($a);
	hello($a);

    return trim($a);
}

/** @template T of mixed
 * @param T $a
 * @return (T is string ? string : T)
 */
function hello2(mixed $a): mixed
{
    if (!is_string($a)) {
        return $a;
    }

    return trim($a);
}

Code snippet that reproduces the problem

https://phpstan.org/r/efe015eb-2eb6-44fe-a699-4034c576705c

Expected output

I'm not sure if I must do something different or if it's an error in phpstan

Did PHPStan help you today? Did it make you happy in any way?

yes thanks