业务需求创造了一个bug，需要修复，经过老长一段时间的折腾，定位出原因了，解决方案，重写核心代码，放开限制。

3下5除2开开心心地进行模块重写，启用模块，升级，并编译，结果报了Fatal error:  Uncaught Error: Undefined constant和PHP Fatal error:  Class 'Multiline' not found，如下

 sudo bin/magento setup:di:compile
Compilation was started.

Repositories code generation... 1/7 [====>-----------------------]  14% 2 secs 101.0 MiBPHP Fatal error:  Uncaught Error: Class 'Sna\Eav\Model\Attribute\Data\Magento\Eav\Model\Attribute\Data\Multiline' not found in /var/www/html/m2.34-jp/app/code/Sna/Eav/Model/Attribute/Data/Multiline.php:15
Stack trace:
#0 /var/www/html/m2.34-jp/vendor/composer/ClassLoader.php(444): include()
#1 /var/www/html/m2.34-jp/vendor/composer/ClassLoader.php(322): Composer\Autoload\includeFile()
#2 [internal function]: Composer\Autoload\ClassLoader->loadClass()
#3 [internal function]: spl_autoload_call()
#4 /var/www/html/m2.34-jp/setup/src/Magento/Setup/Module/Di/Code/Reader/ClassesScanner.php(133): class_exists()
#5 /var/www/html/m2.34-jp/setup/src/Magento/Setup/Module/Di/Code/Reader/ClassesScanner.php(117): Magento\Setup\Module\Di\Code\Reader\ClassesScanner->includeClasses()
#6 /var/www/html/m2.34-jp/setup/src/Magento/Setup/Module/Di/Code/Reader/ClassesScanner.php(88): Magento\Setup\Module\Di\Code\Reader\ClassesScanner->extract()
#7 /var/www/html/m2.34-jp/setup/src/Magento/Setup/Module/Di/App/Task/Operation/RepositoryGenerator.php(61 in /var/www/html/m2.34-jp/app/code/Sna/Eav/Model/Attribute/Data/Multiline.php on line 15

找来找去终于知道了原因，开头的class Multiline extends Magento\Eav\Model\Attribute\Data\Multiline的写法要改一下，extends后面的Magento要以\开头，即是\Magento，完整如下：

再次编译，已经不报相同的错误了，但却报了另外一个错误，如下

namespace \Silk\Eav\Model\Attribute\Data;

// use Magento\Framework\App\RequestInterface;

/**
 * EAV Entity Attribute Multiply line Data Model
 *
 * @author      Magento Core Team <[email protected]>
 */
class Multiline extends \Magento\Eav\Model\Attribute\Data\Multiline
{
   /*..省略...*/
}

sudo php7.2 -dmemory_limit=4G bin/magento setup:di:compile

Compilation was started.
Repositories code generation... 1/7 [====>-----------------------]  14% < 1 sec 104.5 MiBPHP Fatal error:  Uncaught Error: Undefined constant 'Silk\Eav\Model\Attribute\Data' in /var/www/html/m2.34-jp/app/code/Sna/Eav/Model/Attribute/Data/Multiline.php:6
Stack trace:
#0 /var/www/html/m2.34-jp/vendor/composer/ClassLoader.php(444): include()
#1 /var/www/html/m2.34-jp/vendor/composer/ClassLoader.php(322): Composer\Autoload\includeFile('/var/www/html/m...')
#2 [internal function]: Composer\Autoload\ClassLoader->loadClass('Sna\\Eav\\Model\\...')
#3 [internal function]: spl_autoload_call('Sna\\Eav\\Model\\...')
#4 /var/www/html/m2.34-jp/setup/src/Magento/Setup/Module/Di/Code/Reader/ClassesScanner.php(133): class_exists('\\Sna\\Eav\\Model...')
#5 /var/www/html/m2.34-jp/setup/src/Magento/Setup/Module/Di/Code/Reader/ClassesScanner.php(117): Magento\Setup\Module\Di\Code\Reader\ClassesScanner->includeClasses(Array, '/var/www/html/m...')
#6 /var/www/html/m2.34-jp/setup/src/Magento/Setup/Module/Di/Code/Reader/ClassesScanner.php(88): Magento\Setup\Module\Di\Code\Reader\ClassesScanner->extract(Object(RecursiveIteratorIterator in /var/www/html/m2.34-jp/app/code/Sna/Eav/Model/Attribute/Data/Multiline.php on line 6
刚入坑，又掉坑

这，，入坑刚起来又直接掉坑，好吧，再细看一下文件代码的报错说明，namespace里面以\开头了，尝试去除，再重新编译，可算正常了。正常的代码如下：

#namespace \Silk\Eav\Model\Attribute\Data;
#不能以\开头，编译时会出错

namespace Silk\Eav\Model\Attribute\Data;

// use Magento\Framework\App\RequestInterface;

/**
 * EAV Entity Attribute Multiply line Data Model
 *
 * @author      Magento Core Team <[email protected]>
 */
class Multiline extends \Magento\Eav\Model\Attribute\Data\Multiline
{
   /*..省略...*/
}
总结：Magneto2模块在功能重写时，namespace不要以\开头，而重写继承的模块在extends后面的Magento模块，则要以\开头，这样进行重写时，才能找到正常的class，编译才会正常