Cast the index length to int in the MySqlSchemaManager class

lib/Doctrine/DBAL/Schema/MySqlSchemaManager.php

@@ -68,7 +68,7 @@ class MySqlSchemaManager extends AbstractSchemaManager
             } elseif (strpos($v['index_type'], 'SPATIAL') !== false) {
                 $v['flags'] = ['SPATIAL'];
             }
-            $v['length'] = $v['sub_part'] ?? null;
+            $v['length'] = isset($v['sub_part']) ? (int) $v['sub_part'] : null;
 
             $tableIndexes[$k] = $v;
         }

tests/Doctrine/Tests/DBAL/Functional/Schema/MySqlSchemaManagerTest.php

@@ -107,6 +107,19 @@ class MySqlSchemaManagerTest extends SchemaManagerFunctionalTestCase
         self::assertTrue($indexes['s_index']->hasFlag('spatial'));
     }
 
+    public function testIndexWithLength() : void
+    {
+        $table = new Table('index_length');
+        $table->addColumn('text', 'string', ['length' => 255]);
+        $table->addIndex(['text'], 'text_index', [], ['lengths' => [128]]);
+
+        $this->schemaManager->dropAndCreateTable($table);
+
+        $indexes = $this->schemaManager->listTableIndexes('index_length');
+        self::assertArrayHasKey('text_index', $indexes);
+        self::assertSame([128], $indexes['text_index']->getOption('lengths'));
+    }
+
     /**
      * @group DBAL-400
      */