diagnosing "Composer: Your requirements could not be resolved to an installable set of packages" issues

I tried updating brianium/paratest from ^6.6 to the latest version of ^7.0 and now, when I try to run composer install, I get this message:

Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Root composer.json requires vimeo/psalm ^4.29 -> satisfiable by vimeo/psalm[4.29.0, 4.30.0].
    - brianium/paratest[v7.0.0, ..., v7.0.1] require phpunit/phpunit ^10.0.3 -> satisfiable by phpunit/phpunit[10.0.3, 10.0.4].
    - phpunit/phpunit[10.0.3, ..., 10.0.4] require sebastian/diff ^5.0 -> satisfiable by sebastian/diff[5.0.0].
    - Conclusion: don't install sebastian/diff 5.0.0 (conflict analysis result)
    - Root composer.json requires brianium/paratest ^7.0 -> satisfiable by brianium/paratest[v7.0.0, v7.0.1].

Error: Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Root composer.json requires vimeo/psalm ^4.29 -> satisfiable by vimeo/psalm[4.29.0, 4.30.0].
    - brianium/paratest[v7.0.0, ..., v7.0.1] require phpunit/phpunit ^10.0.3 -> satisfiable by phpunit/phpunit[10.0.3, 10.0.4].
    - phpunit/phpunit[10.0.3, ..., 10.0.4] require sebastian/diff ^5.0 -> satisfiable by sebastian/diff[5.0.0].
    - Conclusion: don't install sebastian/diff 5.0.0 (conflict analysis result)
    - Root composer.json requires brianium/paratest ^7.0 -> satisfiable by brianium/paratest[v7.0.0, v7.0.1].

Error: Process completed with exit code 2.

Here's my composer.json:

    "require": {
        "php": ">=8.1"
    },
    "require-dev": {
        "ext-xml": "*",
        "brianium/paratest": "^7.0",
        "friendsofphp/php-cs-fixer": "^3.12",
        "php-parallel-lint/php-parallel-lint": "^1.3",
        "squizlabs/php_codesniffer": "^3.7",
        "vimeo/psalm": "^4.29"
    },

I'm not requiring sebastian/diff myself so I can't remove it as a requirement. Per the output I guess phpunit/phpunit is but if it were the only thing requiring sebastian/diff then I don't see how there'd be a conflict. So it seems like something else must be requiring sebastian/diff, albeit a different version, but I have no idea what.

I suppose I could look at each my deps and consider upgrading each one of them, as well, but idk... that could be a huuuge PITA if I had a lot of deps. Is there no better way to figure out what's going on?

I did do that for my specific composer.json and it looks like PHP-CS-Fixer/PHP-CS-Fixer (even the latest version of v3.14.3) requires ^4.0 whereas phpunit/phpunit 10.x requires ^5.0 so I guess that's the issue but I feel like I shouldn't need to have to look at each individual repo's composer.json to figure that out.

Just have a look at the requirements of vimeo/psalm. v4 requires sebastian/diff in v3 or v4, as you can see on https://packagist.org/packages/vimeo/psalm#4.29.0, and this is obviously incompatible with PHPUnit v10, as this requires sebastian/diff in v5.

You could check if your application supports running Psalm v5. And if not, this is a great example for why you should not install tools like Psalm using Composer

Also, next time you run such an update, composer why-not could help. In your example, composer why-not brianium/paratest 7.0.1 could have pointed out the incompatible package