Leverages #42149

We need extensive doc on the topic for sure, a whole new chapter.

DebugClassLoader allows patching an app or a lib by going through these steps:

• require symfony/error-handler if not already there
• run composer install -q --optimize-autoloader
• copy/paste the below script to the root of the app/lib, in patch-types.php
• run the script with SYMFONY_PATCH_TYPE_DECLARATIONS='force=phpdoc' php patch-types.php

SYMFONY_PATCH_TYPE_DECLARATIONS can be set to:

• 'force=phpdoc' to copy @return annotations from parent classes, to express that the next major version of that lib is going to add a native return types;
• 'force=1' to turn @return annotations into native return types, but only on tests/private/final/internal methods;
• 'force=2' to turn @return annotations into native return types, for all possible methods.

<?php

if (false === getenv('SYMFONY_PATCH_TYPE_DECLARATIONS')) {
    echo "Please define the SYMFONY_PATCH_TYPE_DECLARATIONS env var when running this script.\n";
    exit(1);
}

$loader = require __DIR__.'/vendor/autoload.php';

Symfony\Component\ErrorHandler\DebugClassLoader::enable();

foreach ($loader->getClassMap() as $class => $file) {
    switch (true) {
        case false !== strpos($file = realpath($file), '/vendor/'):
        case false !== strpos($file, '/src/path-to-exclude/'): // add as many as required
            continue 2;
    }

    class_exists($class);
}

Symfony\Component\ErrorHandler\DebugClassLoader::checkClasses();