Quelling default completions for homonymous commands

["Bart Schaefer" <schaefer@xxxxxxxxxxxxxxxx>]

I have a shell function named "show" that I've used for years.  It has
nothing to do with the MH command "show", but I've just noticed that zsh
complains to stderr "_mh:1: command not found: mhpath" when I complete for
my "show" function.

This is foremost a bug in _mh, because completion functions should not
invoke external commands without a strategic 2>/dev/null.  However, I'd
rather skip the MH completion attempt entirely.

Obviously I can "compdef -d show" somewhere in my startup files, but I
wonder whether there ought to be some more generalized way to avoid even
loading the completion functions for commands that are not installed.

Thoughts?