I just grabbed the latest Magento 2 from source, and suddenly PHP was complaining about a method declaration
PHP Fatal error: Declaration of MagentoCoreModelUrlProxy::setRequest() must be compatible with MagentoUrlInterface::setRequest(MagentoAppRequestInterface $request) in /Users/alanstorm/Documents/apache-git/github-magento2-10-4-2013/magento2/var/generation/Magento/Core/Model/Url/Proxy.php on line 8
Even stranger, the class
MagentoCoreModelUrlProxy is no longer a part of the latest Magento source code.
Taking a closer look at the error revealed the problem
It looks like Magento 2 is doing some sort of code generation/caching in the
var/generation folder. Removing this folder (and it’s generated classes) appears to have cleared up the problem.