[Windows + MSYS2/fish + NeoVIM] error: Error reading script file '/s' when running `PlugUpdate`

[tigerinus]

Run nvim -c PlugUpdate and got this:

Please give some hints on where to look further about this issue.

---

" vim-plug
call plug#begin('~/.vim/plugins')
Plug 'tpope/vim-sensible'               " settings that make sense
Plug 'joshdick/onedark.vim'             " color theme
Plug 'airblade/vim-gitgutter'           " reflect git changes per line
Plug 'sheerun/vim-polyglot'             " syntax highlighting for languages
Plug 'scrooloose/syntastic'             " syntax checker for languages
Plug 'romgrk/barbar.nvim'               " tabline
Plug 'vim-airline/vim-airline'          " statusline
Plug 'vim-airline/vim-airline-themes'   " status bar theme
Plug 'godlygeek/tabular'                " text filtering and alignment
Plug 'chiel92/vim-autoformat'           " code formatting
Plug 'kyazdani42/nvim-web-devicons'     " icons

call plug#end()

NVIM v0.8.3
Build type: RelWithDebInfo
LuaJIT 2.1.0-beta3
Compiled by runneradmin@fv-az171-224

Features: -acl +iconv +tui
See ":help feature-compile"

   system vimrc file: "$VIM\sysinit.vim"
  fall-back for $VIM: "C:/Program Files (x86)/nvim/share/nvim"

Run :checkhealth for more info

• Type:
  • [?] Bug
  • [ ] Enhancement
  • [ ] Feature Request
  • [x] Question
• OS:
  • [ ] All/Other
  • [ ] Linux
  • [ ] macOS
  • [x] Windows
• Vim:
  • [ ] Terminal Vim
  • [ ] GVim
  • [x] Neovim
• Git:
  • [x] MSYS2
• Shell:
  • [x] fish

Workaround

SHELL= nvim -c PlugUpdate         

[tigerinus]

I reproduced the issue on a different Windows machine

NVIM v0.8.2
Build type: RelWithDebInfo
LuaJIT 2.1.0-beta3
Compiled by runneradmin@fv-az28-353

Features: -acl +iconv +tui
See ":help feature-compile"

   system vimrc file: "$VIM\sysinit.vim"
  fall-back for $VIM: "C:/Program Files (x86)/nvim/share/nvim"

Run :checkhealth for more info

[tigerinus]

I got the same error message when running fish /s whatever

Somewhere in your .vim must be calling $SHELL /s ... or something like that.