RE: [PATCH] Add new function to test whether a key is present in a hash table.

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

> > Such a function is useful because in Emacs Lisp, 'gethash'
> > cannot return whether the key is present as in Common Lisp,
> > and using 'gethash' alone to test for presence is nontrivial.
> 
> We could also implement it in Elisp:
> (defun hash-table-contains-p (key table)
>   (let ((x '(:hash-table-contains-p)))
>     (not (eq x (gethash key table x)))))

Yes, as I said:
  Everyone was coding their own (in Lisp).

using a unique cons, uninterned symbol, or
some other unique object.

Philipp used an uninterned symbol:
(let ((uniq-symb  '#:void))
  (not (eq uniq-symb (gethash key table uniq-symb))))

I (like you) used a unique cons:
(let ((uniq-cons  (cons 1 1)))
  (not (eq uniq-cons (gethash key table uniq-cons))))

But isn't it better to define this in C?

https://debbugs.gnu.org/cgi/bugreport.cgi?bug=28753