$this->resolver    ->setDefault('connections',staticfunction (OptionsResolver$resolver) {// nested option$resolver            ->setPrototype(true)// <- the new method            ->setRequired('table')            ->setDefaults(['user' =>'root','password' =>null]);    });

$this->resolver->resolve(['connections' => ['default' => [// <- the array index "default" is optional and validation free'table' =>'default',        ],'custom' => ['user' =>'foo','password' =>'pa$$','table' =>'symfony',        ],    ],])

['connections' => ['default' => ['user' =>'root','password' =>null,'table' =>'default',        ],'custom' => ['user' =>'foo','password' =>'pa$$','table' =>'symfony',        ],    ],]