I think#34757 fixed the issue already.

Wouldn't this cause issues in case multiple Composer class loaders are registered in memory at the time the container is dumped ? This could happen for instance if you run Behat from a phar (which uses the composer ClassLoader inside the phar) or a global composer install (or whatever else custom install also using composer) and the Behat run has to refresh the container cache. This might also happen when using simple-phpunit rather than Behat btw, because it also uses a composer-based installation for PHPUnit but independent from the vendor folder
That's why there was a check to ensure the autoload file being found is actually the one belonging to the project (this check might have been broken, but it is necessary).

@nicolas-grekas do we have a test asserting the output of the dumper for the preload.php file ?

@stof yes we do

Ohh indeed, My Patch was totally wrong, because the issue was somewhere else

when project is installed in/foo it doesn't work. BUT when it' installed in/foo/bar it's fine.

I update the patch according to the original issue

fixed by#34757