Classnames 'Void' inside vendor/magento/

Here is how to fix it:

1. edit vendor/magento/module-sales/etc/webapi.xml line 112:

   - <route url="/V1/invoices/:id/void" method="POST">
   + <route url="/V1/invoices/:id/voidInvoice" method="POST">
   

2. edit vendor/magento/module-sales/Block/Adminhtml/Order/Invoice/View.php line 242:

   - return $this->getUrl('sales/*/void', ['invoice_id' => $this->getInvoice()->getId()]);
   + return $this->getUrl('sales/*/voidInvoice', ['invoice_id' => $this->getInvoice()->getId()]);
   

3. edit vendor/magento/module-sales/Block/Adminhtml/Order/Creditmemo/View.php line 183:

   - return $this->getUrl('sales/*/void', ['creditmemo_id' => $this->getCreditmemo()->getId()]);
   + return $this->getUrl('sales/*/voidCreditmemo', ['creditmemo_id' => $this->getCreditmemo()->getId()]);
   

4. edit vendor/magento/module-sales/Controller/Adminhtml/Order/Invoice/Void.php line 9:

   - class Void extends \Magento\Sales\Controller\Adminhtml\Invoice\AbstractInvoice\View
   + class VoidInvoice extends \Magento\Sales\Controller\Adminhtml\Invoice\AbstractInvoice\View
   

5. edit vendor/magento/module-sales/Controller/Adminhtml/Order/Creditmemo/Void.php line 10:

   - class Void extends \Magento\Backend\App\Action
   + class VoidCreditmemo extends \Magento\Backend\App\Action
   

6. rename

   vendor/magento/module-sales/Controller/Adminhtml/Order/Creditmemo/Void.php

   to

   vendor/magento/module-sales/Controller/Adminhtml/Order/Creditmemo/VoidCreditmemo.php

7. rename

   vendor/magento/module-sales/Controller/Adminhtml/Order/Invoice/Void.php

   to

   vendor/magento/module-sales/Controller/Adminhtml/Order/Invoice/VoidInvoice.php

taken from https://mage2.pro/t/topic/2433/2

I don't think Magento 2 is yet compatible with php 7.1-alpha, but this is an alarm signal.
I think you should report this on github.
It could be an issue in the near future,

Instead of modifying the core magento vendor files to resolve the issue it would make more sense to use the right php version. Depending on what you are using, it should be easy enough to set a BASH configuration for a particular php bin location.