vim-patch:8.2.1801: undo file not found when using ":args" or ":next"

Problem:    Undo file not found when using ":args" or ":next".
Solution:   Handle like editing another file. (closes vim/vim#7072)
https://github.com/vim/vim/commit/55b419b871dd35f5b05dd2aed65f14461b493ba9

2 files changed, 26 insertions, 2 deletions

diff --git a/src/nvim/ex_cmds.c b/src/nvim/ex_cmds.c
index 9be6adcd61..bb4e92efc0 100644
--- a/src/nvim/ex_cmds.c
+++ b/src/nvim/ex_cmds.c
@@ -2497,8 +2497,12 @@ int do_ecmd(
       new_name = NULL;
     }
     set_bufref(&bufref, buf);
-    if (p_ur < 0 || curbuf->b_ml.ml_line_count <= p_ur) {
-      // Save all the text, so that the reload can be undone.
+
+    // If the buffer was used before, store the current contents so that
+    // the reload can be undone.  Do not do this if the (empty) buffer is
+    // being re-used for another file.
+    if (!(curbuf->b_flags & BF_NEVERLOADED)
+        && (p_ur < 0 || curbuf->b_ml.ml_line_count <= p_ur)) {
       // Sync first so that this is a separate undo-able action.
       u_sync(false);
       if (u_savecommon(0, curbuf->b_ml.ml_line_count + 1, 0, true)
diff --git a/src/nvim/testdir/test_undo.vim b/src/nvim/testdir/test_undo.vim
index 1bdc9bfd4b..3b66071d6d 100644
--- a/src/nvim/testdir/test_undo.vim
+++ b/src/nvim/testdir/test_undo.vim
@@ -392,6 +392,26 @@ func Test_rundo_errors()
   call delete('Xundofile')
 endfunc
 
+func Test_undofile_next()
+  set undofile
+  new Xfoo.txt
+  execute "norm ix\<c-g>uy\<c-g>uz\<Esc>"
+  write
+  bwipe
+
+  next Xfoo.txt
+  call assert_equal('xyz', getline(1))
+  silent undo
+  call assert_equal('xy', getline(1))
+  silent undo
+  call assert_equal('x', getline(1))
+  bwipe!
+
+  call delete('Xfoo.txt')
+  call delete('.Xfoo.txt.un~')
+  set undofile&
+endfunc
+
 " Test for undo working properly when executing commands from a register.
 " Also test this in an empty buffer.
 func Test_cmd_in_reg_undo()