http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=32282&start=0

Can we mark this one for 4.0? It will break quite a few extensions where the custom fields will extend core fields...

Can't confirm that this breaks 3rd party extensions.
I extend quite a few of those in my own component and it still works without issues.
After all we should not directly load the file we extend but use JFormHelper::loadFieldClass('list') and this function already tries to autoload a class. After this PR, this call even is no longer needed imho.

@Hackwar Did you have something break or is this just a guess based on code review?

This is a guess on code that I've seen in the past. I know that we shouldn't do a straight require_once(), but expect developers to do exactly that most of the time.

@Hackwar JFormHelper::loadFieldClass() is part of Joomla since at least 1.7. So if developers don't use the provided API and still use require_once() instead, they're on their own.
We could never move those files around if we count that as a BC break as we can't deprecate require_once for obvious reasons

No, of course not, but with 4.0 we are allowed to have a b/c break, so we can simply do it then and point to our disclaimer. I'm not very emotional about this, but I know that it will cause numerous issues.

It's not a BC break since the API doesn't break.
Also 4.0 will likely not be allowed to break BC at will. So it will not be much different.

I've moved this from being against master to being against 3.3 dev branch in #3116