setUp () 方法要设置返回值类型声明

    public function setUp(): void
    {
        parent::setUp();

        $this->user = factory(User::class)->create();
    }

如果不加 : void 的话会报错
PHP Fatal error: Declaration of Tests\Feature\TopicApiTest::setUp() must be compatible with Illuminate\Foundation\Testing\TestCase::setUp(): void in /home/vagrant/code/bbs/tests/Feature/TopicApiTest.php on line 12
参考