bug#29165: 26.0.90; can't use some code byte-compiled under emacs 24

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

> It's not the byte compiler specifically, the defun* macro (aka cl-defun)
> expands &key into &rest, so the expansion has &optional &rest.  At any
> rate, using &optional is meaningless there, so the solution is just to
> remove it.
> 
> We should perhaps put something about throwing error on '&option &rest'
> into NEWS though.

I don't understand.  In Common Lisp it is perfectly correct
to use both &optional and &rest.  If Emacs's emulation of
CL's `defun' is defective in this regard, is that not a bug?

https://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node64.html

Or are you saying that there was a bug in Emacs 24's handling
of the CL `defun' emulation, where it expanded &key into
&rest, producing multiple &optional or &rest, and that now
this has been fixed - hence Emacs 24 byte-code does not work
in Emacs 26?

If you do add something to NEWS, please make clear just what
the problem is/was.  Does Emacs now correctly emulate CL
`defun' wrt &* keywords?  Is it an error to repeat &optional
or &rest?  If not, what is the behavior in that case?

Whatever the explanation is, please make it very clear in
NEWS.  Thx.