fix: Override merging behaviour

system/Config/BaseConfig.php

@@ -246,8 +246,8 @@ protected function registerProperties()
             if (static::$discovering) {
                 throw new ConfigException(
                     'During Auto-Discovery of Registrars,'
-                    . ' "' . static::class . '" executes Auto-Discovery again.'
-                    . ' "' . clean_path(static::$registrarFile) . '" seems to have bad code.',
+                        . ' "' . static::class . '" executes Auto-Discovery again.'
+                        . ' "' . clean_path(static::$registrarFile) . '" seems to have bad code.',
                 );
             }
 
@@ -291,7 +291,7 @@ protected function registerProperties()
 
             foreach ($properties as $property => $value) {
                 if (isset($this->{$property}) && is_array($this->{$property}) && is_array($value)) {
-                    $this->{$property} = array_merge($this->{$property}, $value);
+                    $this->{$property} = array_merge_recursive($this->{$property}, $value);
                 } else {
                     $this->{$property} = $value;
                 }

tests/_support/Config/TestRegistrar.php

@@ -27,6 +27,9 @@ public static function RegistrarConfig()
                 'first',
                 'second',
             ],
+            'baz' => [
+                'bar' => 'ber',
+            ],
         ];
     }
 }

tests/system/Config/BaseConfigTest.php

@@ -270,6 +270,9 @@ public function testRegistrars(): void
         $this->assertSame('bar', $config->foo);
         // add to an existing array property
         $this->assertSame(['baz', 'first', 'second'], $config->bar);
+
+        // add to an existing nested array property
+        $this->assertSame(['foo', 'bar' => ['bir', 'bur', 'ber']], $config->baz);
     }
 
     public function testBadRegistrar(): void

tests/system/Config/fixtures/RegistrarConfig.php

@@ -19,4 +19,11 @@
     public $bar = [
         'baz',
     ];
+    public array $baz = [
+        'foo',
+        'bar' => [
+            'bir',
+            'bur',
+        ],
+    ];
 }