commit https://github.com/vim/vim/commit/f5a39447a8ebe162ee62caa2ee502cd0e65eecaa Author: Bram Moolenaar <Bram@vim.org> Date: Tue Aug 16 21:04:41 2016 +0200 patch 7.4.2219 Problem: Recursive call to substitute gets stuck in sandbox. (Nikolai Pavlov) Solution: Handle the recursive call. (Christian Brabandt, closes https://github.com/vim/vim/issues/950) Add a test.

" Test feedkeys() function.

func Test_feedkeys_x_with_empty_string()
  new
  call feedkeys("ifoo\<Esc>")
  call assert_equal('', getline('.'))
  call feedkeys('', 'x')
  call assert_equal('foo', getline('.'))

  " check it goes back to normal mode immediately.
  call feedkeys('i', 'x')
  call assert_equal('foo', getline('.'))
  quit!
endfunc