bug#35546: 27.0.50; setf return value for new alist entries is wrong

[npostavs]

Michael Heerdegen <michael_heerdegen@web.de> writes:

> struggling with 'logand', because I don't understand what it is doing.
> Not only that "it should arguably fail when trying to set a value
> outside of the mask" as described in the file header, I also don't see
> how the used formula makes more sense than setting the place to just V.

I think the rationale goes like this:

Suppose you want to *get* the bottom 4 bits of PLACE, you do

    (logand PLACE #x0F)

    ;; Example:
    (let ((var #xABCD))
      (logand var #x0F)) ;=> #xD

Suppose you want to *set* the bottom 4 bits of PLACE, you do

    (setf (logand PLACE #x0F) VALUE)

    ;; Example:
    (let ((var #xABCD))
      (setf (logand var #x0F) 9)
      var) ;=> #xABC9