How do I fix "E302: Could not rename swap file" when using neovim as my MANPAGER with a non-default swap directory?

Question

I'd like to use neovim to view man pages. To accomplish this, I have the following set in my .bashrc file:

export MANPAGER='nvim +Man!'

On my MacOS system this gives me no problems. But on my Linux system, opening a man page eventually comes up in neovim, but on startup for each man page brings up the following annoying message for about a second or two:

Error detected while processing function man#init_pager:
line   20:                                                                                                                                                                                                                                                                                    
E302: Could not rename swap file

I've found that this is related to putting my swap files in ~/.vim/swap because I prefer them there rather than polluting my working tree:

" Where to store swap files.  By default, they will go into ~/.vim/swap, but                                    
" if that doesn't work, they will go in cwd.                                    
set directory=~/.vim/swap,.                                     

I only see this with neovim, not vim.

Is there a way to get these two settings (using neovim as my MANPAGER and setting an alternate swap location) to work together?

---

In case it's helpful, this can be reproduced rather minimally like so:

docker pull ubuntu
docker run -it --name test_nvim_manpager ubuntu /bin/bash

apt update
apt install -y neovim man
unminimize

mkdir -p ~/.config/nvim
echo 'source ~/.vimrc' > ~/.config/nvim/init.vim

echo "export MANPAGER='nvim +Man\!'" >> ~/.bashrc
echo "set directory=~/.vim/swap,." >> ~/.vimrc
source ~/.bashrc

man ls

---

Update: This now happens with my mac running vim v0.7.2. I suspect that this now happens due to a vim upgrade, but I'm not sure now from what version.

Answer 1

In case it's helpful for someone else, I noticed that the filenames when vim is used as a MANPAGER starts with man://. With this in mind, I avoid switching the swap file location for man pages by wrapping the directive in a condition in my ~/.vimrc file:

" Changing the swap directory for the MANPAGER causes a "E302: Could not                                       
" rename swap file" error. Therefore do not change the swap directory for                                          
" man pages.   
if stridx(@%, 'man://') >= 0                                                                                      
  " Where to store swap files.  By default, they will go into ~/.vim/swap, but                                     
  " if that doesn't work, they will go in cwd.                                                             
  set directory=~/.vim/swap,.                                                                                      
endif