How to autoload another package with own composer - php

Question

EDIT with important note: The package I want to include does not use composer autoload. I'd have to use their hacky one and I want to avoid that.
I know how composer mostly works and I have package that can be a dependency (that's important, I know how to make this work in one project, but that's not what I'm asking).
What I need?
Somebody requires my package
composer require tomas/my-package
It will install
It will autoload my package with PSR-4
It will autoload 3rd party package as well
I have already tried something like this:
"autoload": {
"psr-4": {
"MyPackage\\": "src",
"PHP_CodeSniffer\\": "../../squizlabs/php_codesniffer/src"
}
}
I've tried that in one of my dependencies and it doesn't work :(.
Also, I've already talked to the package author and he doesn't want to use composer autoloading. He prefers his own.
Thanks for any help!

Answer 1

None if these answer are related to problem I desribed, so I try to post best solution so far:
I got inspired in PHPStan, which has feture to autoload directories, that were missed in composer (practically same issue).
I've added RobotLoader:
composer require nette/robot-loader
Then create LegacyCompatibility class with static method:
use Nette\Loaders\RobotLoader;
// ...
public static function autoloadCodeSniffer(): void
{
$robotLoader = new RobotLoader;
$robotLoader->acceptFiles = '*.php';
$robotLoader->setTempDirectory(sys_get_temp_dir() . '/_robot_loader');
$robotLoader->addDirectory(getcwd() . '/vendor/squizlabs/php_codesniffer/src');
$robotLoader->register();
}
And I call it, when needed:
LegacyCompatibility::autoloadCodeSniffer()
Works like a charm. But still opened to better solutions +1 !
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Answer 2

You are doing one big NO-NO in your composer.json file: You are defining autoloading for code that is not in your package.
This will fail, as you found out already.
And I wonder why you are struggling with integrating the PHP Codesniffer, because that package has a working autoloading definition included. All you'd need to do is "require": { "squizlabs/php_codesniffer": "^3.0#RC" } (if you really need the latest 3.x release candidate, otherwise ^2.8.0 would be a better idea).
If you require PHP_Codesniffer instead of somehow referencing it in your autoloading, then Composer will do everything to integrate that package: download it, add it to the autoloader, and run it's code if necessary. And anyone depending on YOUR package will also get this dependency installed.
The only problem you state is that "it doesn't work", which is no sufficient description of a problem because you lack describing what you did in your code, and what error message you got. Also state what you expected to happen, and what happened instead (and how this deviated from your expectation if this isn't obvious).