Bug report: Completion for dynamically named dirs fails when $SUFFIX is not empty

[Marlon Richert <marlon.richert@xxxxxxxxx>]

% 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().