https://github.com/phpstan/phpstan/issues/11250

composer.json

{
    "require-dev": {
        "phpstan/phpstan": "^1.11"
    },
    "autoload": {
        "classmap": ["."]
    }
}

MyClass.php

<?php

declare(strict_types=1);

final class MyClass
{
    use VendorTrait;

    public function __construct(private readonly bool $foo)
    {
        
    }
}

phpstan.neon

parameters:
	level: max
	paths:
		- MyClass.php
	

VendorTrait.php

<?php

declare(strict_types=1);

trait VendorTrait
{
    private readonly bool $foo;

    private function foo(): bool
    {
        return $this->foo;
    }
}