Clash detection is really hard to do right. I need a lot of convincing before it can ever be re-enabled by default.

Clash detection has serious conceptual problems. It is very difficult for emacs to tell if two filenames used by different clients really clash or not - e.g. two clients editing /etc/passwd will clash only if they are on the same machine. In general, clients are going to have separate file name spaces. clash detection requires a directory writable by every client.

In the case where it does work reliably (single-user machine, no network filesystems) is also the case where it's the least useful. A feature that only works intermittently, causes perceptible slowness on all file operations, and fails to scale well is a feature that needs to be explicitly enabled by the user. Keep in mind my vision for the future: someone provides a binary installed XEmacs that is used potentially by everyone worldwide simply by setting their PATH. I implemented such a `non-distribution' of XEmacs in 1993 at IBM. I had users in California, Toronto, and Sweden using the exact same binary. Do we really want these users to deal with mutual clash-detection? Martin I say NO! to enabling global clash-detection.