[cling] crash / compilation exception after undo

[ferdymercury]

Describe the bug

I get a crash with ROOT after a (stupid) test of undo sequences.

Expected behavior

No crash.

To Reproduce

root -l
root [0] int x
(int) 0
root [1] .undo
root [2] x
(int) 0
root [3] .undo
root [4] .undo
root [5] x
input_line_13:2:3: error: use of undeclared identifier 'x'
 (x)
  ^
Error in <HandleInterpreterException>: Error evaluating expression (x).
Execution of your code was aborted.
root [6] x
input_line_15:2:3: error: use of undeclared identifier 'x'
 (x)
  ^
Error in <HandleInterpreterException>: Error evaluating expression (x).
Execution of your code was aborted.
root [7] int x
(int) 0
root [8] int x
(int) 0
root [9] x
(int) 0
root [10] x
(int) 0
root [11] x
(int) 0
root [12] x
(int) 0
root [13] .undo
root [14] x
(int) 0
root [15] .undo 5
root [16] x
input_line_24:2:3: error: use of undeclared identifier 'x'
 (x)
  ^
terminate called after throwing an instance of 'cling::CompilationException'
  what():  Error evaluating expression (x)

Setup

   ------------------------------------------------------------------
  | Welcome to ROOT 6.27/01                        https://root.cern |
  | (c) 1995-2021, The ROOT Team; conception: R. Brun, F. Rademakers |
  | Built for linuxx8664gcc on Jan 12 2022, 10:17:19                 |
  | From heads/master@v6-25-01-2870-gdac9b6398d                      |
  | With c++ (Ubuntu 8.4.0-1ubuntu1~18.04) 8.4.0                     |
  | Try '.help', '.demo', '.license', '.credits', '.quit'/'.q'       |
   ------------------------------------------------------------------

[couet]

Your sequence is a bit long :-) but i see the same on MacOS

[jalopezg-git]

Removing the 6.28/00 milestone; it's low priority, thus postponing.

[ferdymercury]

This works now fine on 6.28/04, it no longer crashes.

root -l
root [0] int x
(int) 0
root [1] .undo
root [2] x
(int) 0
root [3] .undo
root [4] .undo
root [5] x
input_line_12:2:3: error: use of undeclared identifier 'x'
 (x)
  ^
Error in <HandleInterpreterException>: Error evaluating expression (x)
Execution of your code was aborted.
root [6] x
input_line_14:2:3: error: use of undeclared identifier 'x'
 (x)
  ^
Error in <HandleInterpreterException>: Error evaluating expression (x)
Execution of your code was aborted.
root [7] int x
(int) 0
root [8] int x
(int) 0
root [9] x
(int) 0
root [10] x
(int) 0
root [11] x
(int) 0
root [12] x
(int) 0
root [13] .undo
root [14] x
(int) 0
root [15] .undo 5
root [16] x
input_line_23:2:3: error: use of undeclared identifier 'x'
 (x)
  ^
Error in <HandleInterpreterException>: Error evaluating expression (x)
Execution of your code was aborted.
root [17] .q