% cd $(mktemp -d)
% ZDOTDIR=$PWD HOME=$PWD zsh -f
% autoload -Uz compinit
% compinit
% zsh_directory_name() { [[ $1 == c ]] && compadd -S ']' foo }

Given the above setup, when


   1. $LBUFFER is `cd ~[` or `cd ~[fo`,
   2. $RBUFFER is empty, and
   3. I press ^I,
   4. then $LBUFFER becomes `cd ~[foo]`.




   1. $LBUFFER is `cd ~[`,
   2. $RBUFFER is `fo` or `]`, and
   3. I press ^I,
   4. then completion beeps and the buffer remains unchanged.


`functions -t _complete` shows that the problem is caused by
$compstate[context] becoming `tilde` instead of `subscript` as soon as
$SUFFIX is non-empty, which causes _complete() to not call _subscript(),
which is the only point of entry to _dynamic_directory_name().