Breaking: #63437 - Class aliases moved to legacy extension¶
With the switch to namespaced classes in TYPO3 CMS 6.0, a class alias mechanism has been set up to support the old class names as aliases for a while. All those class aliases have been moved to the dedicated extension "compatibility6". This extension will be removed at some point during the development of TYPO3 CMS 7.
Removing the class aliases from the core results in a significant performance gain especially during first load with empty caches.
If extensions still rely on old non-namespaced class names, EXT:compatibility6 can be loaded to keep further backwards compatibility for now - with the side-effect of drained performance.
TYPO3 CMS 7 installations need EXT:compatibility6 loaded if old extensions are used that are still not adapted to the namespaced core classes.
During an upgrade, the "Extension check" of the install tool may find old extensions that still rely on old class names and can unload those. The backend may work again to load EXT:compatibility6, or to unload further extensions that rely on old class names.
Another option is to manually set all extensions that rely on old class names to "inactive" in typo3conf/PackageStates.php, or to set EXT:compatibility6 to "active". If a manual change is done, typo3temp/Cache directory have to be flushed afterwards.