Issue

The installation app contains a JFormFieldLanguage class which has demonstrated previously why we cannot make the core JFormField classes autoloadable (see #3116).

Solution

JForm supports namespacing of classes by a dotted notation in the type attribute of a field's XML declaration. In this case, type="language" expects to load an instance of JFormFieldLanguage (which is a duplicated class). Using namespacing, we can use type="installation.language" to ask JForm to load a class named InstallationFormFieldLanguage.

Additional Changes

Instead of only moving the problem class in the install app, I've instead moved all of the custom JFormField and JFormRule classes in the install app to use the Installation class prefix versus J. This allows us to demonstrate within core this feature of JForm (which truthfully isn't presented very well) and demonstrate that custom field elements can be loaded without the use of addfieldpath and addrulepath attributes in the form XMLs, which should be a preferred behavior in code that is autoloadable (such as the installation application).

Testing Instructions

As this requires testing the installer, the patch tester can't be used (feasibly anyway) to apply this patch. So you'll need to apply the patch using a git client or you can download a full Joomla package via GitHub at https://github.com/mbabker/joomla-cms/archive/InstallLanguageField.zip. Once applied, simply test installing a new Joomla instance; everything should work as before.

Backward Compatibility

Typically, the installation application has been excluded from backward compatibility concerns as the app is only used for new Joomla installations and the code immediately removed from a site. Therefore, class proxies or other practices used in the libraries or components are not applied here.