title: Symfony bundle configuration array node default value merge
body: <p>I have a problem with Symfony4 bundle configuration. I have a configuration class with:</p>
  
  <pre><code>$rootNode
          -&gt;children()
              -&gt;arrayNode('mapping')
                  -&gt;useAttributeAsKey('code')
                  -&gt;prototype('scalar')-&gt;end()
                  -&gt;defaultValue(['default' =&gt; 'test'])
              -&gt;end()
              .....
          -&gt;end();
  </code></pre>
  
  <p>This returns a default configuration like:</p>
  
  <pre><code>array(1) {
     ["default"]=&gt; string(4) "test"
  }
  </code></pre>
  
  <p>But when I add a configuration file:</p>
  
  <pre><code>bundle:
      mapping:
          test: newvalue
          test2: newvalue2
  </code></pre>
  
  <p>Im getting a config:</p>
  
  <pre><code>array(2) {
     ["test"]=&gt; string(8) "newvalue"
     ["test2"]=&gt; string(9) "newvalue2"
  }
  </code></pre>
  
  <p>But I expect to merge these two configs to get:</p>
  
  <pre><code>array(3) {
     ["default"]=&gt; string(4) "test"
     ["test"]=&gt; string(8) "newvalue"
     ["test2"]=&gt; string(9) "newvalue2"
  }
  </code></pre>
  
  <p>How can I set this defaults to be merged with provided configurations? Of course I want to let "default" config to be overridden, but by deafult to be merged.</p>
  
  <p>I cant find any solution on docs <a href="https://symfony.com/doc/current/components/config/definition.html#array-node-options" rel="nofollow noreferrer">https://symfony.com/doc/current/components/config/definition.html#array-node-options</a></p>
  
  <p>Please help :)</p>