Mfcttrf

Q: are there breaking changes to eieio in Emacs 27?

I just upgraded to the Emacs 27 snapshot, and I'm getting a
peculiar break in code that works in previous versions of Emacs.

walkthrough

Here's a toy class and a toy :after method:

(defclass simple-class ()
 ((value
 :initarg :value
 :initform nil
 :documentation "Test slot"))
 "Test class.")

This works:

(simple-class :value "some value") ;; ==> #s(simple-class "some value")

Now I'd like to define an :after method on initialize-instance:

(cl-defmethod initialize-instance :after ((sc simple-class) &key)
 (with-slots (value) sc
 (setf value (upcase value))))

Now it's broken:

(simple-class :value "some value") ;; ==> (error "Keyword argument nil not one of nil")

NB: it's nothing about the content of the :after method: I can
define an empty method and I get the same error.

(cl-defmethod initialize-instance :after ((sc simple-class) &key))

what happened?

So: have there been changes to eieio (specifically,
cl-defmethod) in Emacs 27 that break previous code? Or is there
something bizarre about the snapshot?

So: have there been changes to eieio (specifically, cl-defmethod) in Emacs 27 that break previous code?

It's not a change in eieio or cl-defmethod directly; what's changed is the handling of the &key symbol in cl-def* macros.

(cl-defmethod initialize-instance :after ((sc simple-class) &key))

In Emacs 26 and earlier this produces a function which accepts any number of arguments. In Emacs 27 it produces a function which accepts only one argument (sc). The correct argument list for initialize-instance would be like this:

(cl-defmethod initialize-instance :after ((sc simple-class) &optional slots))

As told by <f1> f initialize-instance RET:

initialize-instance is a compiled Lisp function in ‘eieio.el’.

(initialize-instance THIS &optional SLOTS)