$this->app，没有被定义为什么能访问了

在调用$this->instance('app', $this);之前不能访问，调用之后便可以访问了。点开$this->instance('app', $this);,debug发现核心代码是$this->instances[$abstract] = $instance;，这就看不懂了。
虽然Illuminate\Container\Container定义了__get(),__set(),但其实被没有调用。

有没有理解原理的，不吝赐教。。
感谢！！

// Illuminate\Foundation\Application
protected function registerBaseBindings()
    {
        static::setInstance($this);
        var_dump($this->app); // error
        $this->instance('app', $this);
        var_dump($this->app); // success
        $this->instance(Container::class, $this);
    }

// Illuminate\Container\Container
 public function instance($abstract, $instance)
    {
        $this->removeAbstractAlias($abstract);

        $isBound = $this->bound($abstract);

        unset($this->aliases[$abstract]);

        // We'll check to determine if this type has been bound before, and if it has
        // we will fire the rebound callbacks registered with the container and it
        // can be updated with consuming classes that have gotten resolved here.
        var_dump($this->app); // error
        $this->instances[$abstract] = $instance;
        var_dump($this->app); // success
        if ($isBound) {
            $this->rebound($abstract);
        }
    }

// Illuminate\Container\Container
public function __get($key)
{
  if ($key == 'app') {
 var_dump($key);die;
 // 没有访问
  }
  return $this[$key];
}

public function __set($key, $value)
{
  if ($key == 'app') {
 var_dump($key);die;
 // 没有访问
  }

  $this[$key] = $value;
}