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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Message #26 received at 50219 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: Lars Ingebrigtsen <larsi <at> gnus.org>
Cc: Eric Abrahamsen <eric <at> ericabrahamsen.net>, 50219 <at> debbugs.gnu.org
Subject: Re: bug#50219: 28.0.50; Provide better errors when trying to
 specialize on optional args in generic methods
Date: Sat, 28 Aug 2021 13:41:48 -0400

> (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.

The default value part of args in Common lisp arglists is an
*expression*, so (b string) just means that `b` should take the value of
the `string` variable.

The best I can see (if we keep the CL arglist feature) is to try and see
if that expression looks like a valid CLOS specializer and if so emit
a warning about possible confusion.

I wonder what Common Lisp compilers do.


        Stefan

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