Since upgrading to 5.0.5-1.fc20, my _vim_files autocompletion has been broken:
nightslugs:~% vim <TAB>
_arguments:450: _vim_files: function definition file not found

Re-exec doesn't help this.

Editing /usr/share/zsh/5.0.5/functions/_vim and deleting the:
(${functions[_vim_files]}) ||
prefix before the _vim_files() { ... } definition, makes this work again for me.

(Apologies if the deleted line isn't right; I've removed it now so had to type it from memory. But it's the one that avoids redeclarations if it's already there.)

The only thing I can think of in .zsh/compinstall which would do it is:
zstyle ':completion:*:functions' ignored-patterns '_*'

Any ideas welcome ...