completion-category-overrides

[Chong Yidong]

  *** Completion style can be set per-category `completion-category-overrides'.

  *** Completion of buffers now uses substring completion by default.

Could you explain the reasoning behind using `substring' for buffers?
I'm trying to document this in the manual, and I don't understand it.

At first blush, `completion-category-overrides' seems like the kind of
thing that ought to default to nil, and be left for advanced users who
are sure of what they want.  The fact that buffer completion behaves
more aggressively than completion in the rest of Emacs can be
surprising, as shown by Bug#9598 and Bug#9591.  What makes buffer names
special (as opposed to command names etc.) that they need to be treated
like this by default?

Another issue: Emacs apparently falls back on `completion-styles' if the
styles in `completion-category-overrides' fail.  Why do that?  It makes
it impossible for a user to selectively "turn off" an aggressive
completion style via completion-category-overrides, since Emacs will
always fall back on the aggressive style.  A complete override ought to
be more flexible.