[Messenger] Error when specified default bus is not among the configured

src/Symfony/Bundle/FrameworkBundle/DependencyInjection/Configuration.php

@@ -19,6 +19,7 @@ use Symfony\Component\Asset\Package;
 use Symfony\Component\Config\Definition\Builder\ArrayNodeDefinition;
 use Symfony\Component\Config\Definition\Builder\TreeBuilder;
 use Symfony\Component\Config\Definition\ConfigurationInterface;
+use Symfony\Component\Config\Definition\Exception\InvalidConfigurationException;
 use Symfony\Component\DependencyInjection\Exception\LogicException;
 use Symfony\Component\Form\Form;
 use Symfony\Component\HttpClient\HttpClient;
@@ -1136,6 +1137,10 @@ class Configuration implements ConfigurationInterface
                         ->ifTrue(function ($v) { return isset($v['buses']) && \count($v['buses']) > 1 && null === $v['default_bus']; })
                         ->thenInvalid('You must specify the "default_bus" if you define more than one bus.')
                     ->end()
+                    ->validate()
+                        ->ifTrue(static function ($v): bool { return isset($v['buses']) && null !== $v['default_bus'] && !isset($v['buses'][$v['default_bus']]); })
+                        ->then(static function (array $v): void { throw new InvalidConfigurationException(sprintf('The specified default bus "%s" is not configured. Available buses are "%s".', $v['default_bus'], implode('", "', array_keys($v['buses'])))); })
+                    ->end()
                     ->children()
                         ->arrayNode('routing')
                             ->normalizeKeys(false)

src/Symfony/Bundle/FrameworkBundle/Tests/DependencyInjection/ConfigurationTest.php

@@ -271,6 +271,27 @@ class ConfigurationTest extends TestCase
         ]);
     }
 
+    public function testItErrorsWhenDefaultBusDoesNotExist()
+    {
+        $processor = new Processor();
+        $configuration = new Configuration(true);
+
+        $this->expectException(InvalidConfigurationException::class);
+        $this->expectExceptionMessage('The specified default bus "foo" is not configured. Available buses are "bar", "baz".');
+
+        $processor->processConfiguration($configuration, [
+            [
+                'messenger' => [
+                    'default_bus' => 'foo',
+                    'buses' => [
+                        'bar' => null,
+                        'baz' => null,
+                    ],
+                ],
+            ],
+        ]);
+    }
+
     protected static function getBundleDefaultConfig()
     {
         return [