bug#45879: 28.0.50; [PATCH] missing defvar for keymap-name-history

[Drew Adams <drew.adams@oracle.com>]

> It seems that the new describe-keymaps function in help-fns.el is
> missing a defvar for keymap-name-history. This results in an error
> when using, e.g. Helm as the completing-read function.
> 
> The attached patch adds the defvar to help-fns.el.

FWIW, I don't think it's required for the HIST arg
of `completing-read' (or `read-from-minibuffer') to
be predefined.  It can be any symbol, which is taken
as a variable - no need for a defvar.

Now maybe it can be considered good style to provide
a defvar, especially to provide a doc string.
That's another story.

But it sounds (just a guess) like it's Helm that has
a bug here.  Again though, it's good to provide a
defvar anyway.

E.g. in `emacs -Q':

 (completing-read "aaa: " obarray nil nil nil 'toto)

No error occurs, and `toto' gets populated correctly
as a variable, starting with an initial value of nil,
and regardless of input.

(And the byte-compiler issues no warning either.)

I thought this "feature" was documented, but I don't
find it now in the Elisp manual or doc strings.
Perhaps it's there somewhere.  (Not very important
anyway.)