Incorrect behavior of AddArrowFunctionReturnTypeRector when dealing with a generic void return

[BackEndTea]

Bug Report

Subject	Details
Rector version	last dev-main
Installed as	composer dependency

Minimal PHP Code Causing Issue

See https://getrector.com/demo/829dba24-5560-45e3-a696-d4a438753b08

PHPStan playground: https://phpstan.org/r/28d47df0-e978-43b2-bad5-d568bf6b9135

<?php

final class SkipVoidReturn
{
    public function doNohing(string $a): void
    {
        echo $a;
    }

    public function run()
    {
        fn() => $this->executesCallback(fn() => $this->doNohing('a'));
    }

    /**
     * @template T
     * @param callable(): T $callback
     * @return T
     */
    public function executesCallback(callable $callback): mixed
    {
        return $callback();
    }
}

Responsible rules

• AddArrowFunctionReturnTypeRector

Expected Behavior

Either no change should be made, or a return of null should be added, instead of mixed.
Mixed causes a PHPStan issue, as the return type is incorrect. A return type of null would be (more) correct here.

[samsonasik]

That's expected, null itself is part of mixed and it doesn't cause error as void is auto converted to null when used as return value. I think this should be resolved on phpstan side imo :)