title: How to properly update Symfony3.3 to Symfony4.X?
body: <h2>Explanation:</h2>
  
  <p>I'm unable to update symfony3 to symfony4, it throws a variety of different errors no matter what I try to do to fix them.</p>
  
  <h2>What I've tried:</h2>
  
  <p>I'm following <a href="https://symfony.com/doc/current/setup/upgrade_major.html" rel="nofollow noreferrer">the official documentation</a> regarding major updates and I have ensured I have 0 deprecations.</p>
  
  <p><a href="https://i.stack.imgur.com/UaSwR.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/UaSwR.png" alt="no deprecations"></a></p>
  
  <p>I've updated my <code>composer.json</code> as followed:</p>
  
  <pre><code>{
      "...": "...",
  
      "require": {
          "symfony/symfony": "^4.0",
      },
      "...": "..."
  }
  </code></pre>
  
  <h2>Errors:</h2>
  
  <blockquote>
    <p>Your requirements could not be resolved to an installable set of
    packages.</p>
    
    <p>Problem 1
        - Conclusion: remove symfony/assetic-bundle v2.8.2
        - Conclusion: don't install symfony/assetic-bundle v2.8.2
        - Conclusion: don't install symfony/symfony v4.0.3
        - Conclusion: don't install symfony/symfony v4.0.2
        - Installation request for symfony/assetic-bundle (locked at v2.8.2, required as ^2.8) -> satisfiable by
    symfony/assetic-bundle[v2.8.2].</p>
  </blockquote>
  
  <p>Now, I went ahead and uninstalled symfony/assetic-bundle, but it gives the same error with the following: </p>
  
  <ul>
  <li>"symfony/swiftmailer-bundle"</li>
  <li>"sensio/framework-extra-bundle"</li>
  <li>"sensio/distribution-bundle"</li>
  <li>"incenteev/composer-parameter-handler"</li>
  </ul>
  
  <p>Then when there is nothing left to remove (pretty frustrating at this point), it just says:</p>
  
  <blockquote>
    <p>Problem 1
        - Conclusion: don't install symfony/symfony v4.0.3
        - Conclusion: don't install symfony/symfony v4.0.2
        - Conclusion: don't install symfony/symfony v4.0.1
        - sensio/generator-bundle v3.1.7 requires symfony/yaml
    [...]</p>
    
    <ul>
    <li>Installation request for sensio/generator-bundle (locked at v3.1.7, required as ^3.0) -> satisfiable by sensio/generator-bundle[v3.1.7].</li>
    </ul>
  </blockquote>
  
  <h2>Question:</h2>
  
  <p>How do I correctly solve the errors (shown above) and how do I correctly update symfony to 4.0?</p>
  
  <p><strong>composer.json snippet:</strong></p>
  
  <pre><code>"require": {
          "php": "&gt;=5.5.9",
          "doctrine/doctrine-bundle": "^1.6",
          "doctrine/orm": "^2.5",
          "incenteev/composer-parameter-handler": "^2.0",
          "sensio/distribution-bundle": "^5.0.19",
          "sensio/framework-extra-bundle": "^3.0.2",
          "symfony/monolog-bundle": "^3.1.0",
          "symfony/polyfill-apcu": "^1.0",
          "symfony/swiftmailer-bundle": "^2.3.10",
          "symfony/symfony": "^4.0",
          "twig/twig": "^1.0||^2.0"
      },
      "require-dev": {
          "sensio/generator-bundle": "3.1.7",
          "symfony/phpunit-bridge": "^3.0"
      },
  </code></pre>