bug#61274: 29.0.60; dabbrev-capf signals errors

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Daniel Mendler <mail@daniel-mendler.de>,  61274@debbugs.gnu.org
> Date: Sat, 04 Feb 2023 12:13:27 -0500
> 
> > maybe dabbrev-capf is unsuitable to serve as the value of
> > completion-at-point-functions?
> 
> As the name implies, it's a function designed specifically for use on
> `completion-at-point-functions`.  Maybe dabbrev is not well adapted
> for use within a normal completion UI, but `dabbrev-capf` should do its
> best to obey the rules of `completion-at-point-functions`, so I think
> the behavior Daniel suggests is indeed what `dabbrev-capf` should try
> to do.

Then there's something here that puzzles me: the recipe presented by
Daniel is basically identical to what dabbrev-completion does.  And
yet dabbrev-completion produces different effects when invoked in the
same buffer with the same text at point.  What is responsible for the
difference in behavior?