GNU bug report logs - #50219
28.0.50; Provide better errors when trying to specialize on optional args in generic methods

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From: Lars Ingebrigtsen <larsi <at> gnus.org>
To: Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: Eric Abrahamsen <eric <at> ericabrahamsen.net>, 50219 <at> debbugs.gnu.org
Subject: bug#50219: 28.0.50; Provide better errors when trying to specialize on optional args in generic methods
Date: Sat, 28 Aug 2021 19:12:13 +0200

Stefan Monnier <monnier <at> iro.umontreal.ca> writes:

>> Hm...  So the warning here is correct, I guess.  But perhaps it could
>> also have said something about it being ambiguous syntax (since is also
>> knows the declared parameter list from `cl-defgeneric')...
>
> [ I don't see how the arglist of `cl-defgeneric` would have helped here
>   discover the confusion.  ]

I may be misremembering the semantics of defgeneric, but I thought it
was fine to say:

(cl-defgeneric zot (a &optional b)
  )

But this means that you can only specialise on a -- b is an optional
argument.  So

(cl-defmethod zot ((a integer) &optional (b "foo"))
  (list a b))

is fine and valid -- it's a default value for b.

(cl-defmethod foo ((a integer) &optional (b string))
  ...)

on the other hand, looks like the person who wrote it wanted to
specialise on b, so if the default is something that is a type
specifier, then we could output an additional warning about that.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

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