bug#66165: 30.0.50; Issue with process-get as a place

[Augusto Stoffel]

In some buffer with a process, I get this result:

  (let ((proc (get-buffer-process (current-buffer))))
    (process-put proc 'test 0)
    (cl-incf (process-get proc 'test))
)

  => (test 1)

I would expect of course to back just a number, as with a regular plist:

  (let ((place '(test 0))) (cl-incf (plist-get place 'test)))

  => 1

This happens because process-put doesn't follow the convention of
returning VALUE.  If it's too late to change that, perhaps process-get
needs a special implementation as a generalized variable.