Laravel 5.8在setUp方法的返回类型中添加了void typehint. 因此，您必须像这样声明:

Laravel 5.8 added the void typehint to the return type of the setUp method.
So you have to declare that like this:

public function setUp(): void
{
    // you should also call parent::setUp() to properly boot
    // the Laravel application in your tests
    $this->instance = new MyService;
}

请注意在函数参数后的: void，以声明该函数的返回类型

Note the : void after the function arguments to state the return type of that function