Error in C++ mode with Emacs 27.0.90

[Angelo Graziosi <angelo.g0@libero.it>]

I found an error with C++ mode which I can reproduce with this init.el:

-------------------------------------------
$ cat init.el

;; C/C++ modes
(defun my-c-mode ()
  "My customization for `c-mode' and `c++-mode'."
  (interactive)

  ;; No indent for open bracket
  (c-set-offset 'substatement-open 0)

  ;; Add index of func. to menu bar
  (imenu-add-to-menubar "Functions")
  )

;; c++-mode
(add-hook 'c++-mode-hook 'my-c-mode)

(setq imenu-auto-rescan t)

;; The default is 60000
(setq imenu-auto-rescan-maxout 500000)

;; Show in which function is the cursor
(which-function-mode 1)
-------------------------------------------

(maybe it can be reduced...) and this test case:

-----------------------------------
$ cat foobar.cpp
int main()
{
  return 0;
}
-----------------------------------

When I visit it with C-x C-f, I get this error in minibuffer:

Error in menu-bar-update-hook (imenu-update-menubar): (wrong-type-argument sequencep #<marker at 1 in foobar.cpp>)

The error disappears if I add a space before 'int main()', i.e. with ' int main()'

I have seen that both on GNU/Linux and Windows builds of 27.0.70.

Ciao, 
  Angelo.