bug#64656: 29.0.91; Doc of minibuffer histories and completing-read - automatic addition of completions to DEFAULT list

[Juri Linkov <juri@linkov.net>]

>> > The problem reported by this bug report is that the order is random by
>> > `C-h v M-n M-n' because it uses obarray.
>
> No.  That is NOT the problem reported.  The problem
> reported is:
>
> 1. Initial completions are added to `M-n' automatically,
>    instead of just by a user adding them explicitly to
>    DEFAULTS.
>
> 2. Given that BUG, _IF_ it won't be fixed, then the doc
>    should at least make users aware of it and mention
>    how to work around it.
>
> Fixing #1 is preferable.  Fixing #2 is minimal damage
> control, IF you won't fix #1.

#1 can be fixed only partially for some popular obarray completions.
So here is a patch that fixes #1 partially and #2 completely.

diff --git a/lisp/simple.el b/lisp/simple.el
index f86b3f9e208..02005e3b4f9 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -2989,11 +2989,17 @@ minibuffer-default-add-done
 (defun minibuffer-default-add-completions ()
   "Return a list of all completions without the default value.
 This function is used to add all elements of the completion table to
-the end of the list of defaults just after the default value."
+the end of the list of defaults just after the default value.
+When you don't want to add initial completions to the default value,
+you can use either `minibuffer-setup-hook' or `minibuffer-with-setup-hook'
+to set the value of `minibuffer-default-add-function' to nil."
   (let ((def minibuffer-default)
-	(all (all-completions ""
-			      minibuffer-completion-table
-			      minibuffer-completion-predicate)))
+        ;; Avoid some popular completions with undefined order
+        (all (unless (memq minibuffer-completion-table
+                           `(help--symbol-completion-table ,obarray))
+               (all-completions ""
+                                minibuffer-completion-table
+                                minibuffer-completion-predicate))))
     (if (listp def)
 	(append def all)
       (cons def (delete def all)))))