bug#50219: 28.0.50; Provide better errors when trying to specialize on optional args in generic methods

[Eric Abrahamsen]

Lars Ingebrigtsen <larsi@gnus.org> writes:

> Eric Abrahamsen <eric@ericabrahamsen.net> writes:
>
>> - the manual section on "Generic Functions" could say explicitly that
>>   you can't specialize on optional arguments
>> - `eval'ling the `defmethod' form above could raise an error directly
>> - the compiler could say more explicitly what the problem is
>>
>> I'd favor all three of these changes!
>
> Me, too.

Here's a very simply manual patch.

I squinted long and hard at cl-generic.el, and I'm not confident about where
we'd put the error. In `cl--generic-lambda'?

>> Happy to implement what I can (maybe not the compiler part).
>
> I've added Stefan to the CCs; perhaps he has some comments.

He has already (kindly) made fun of me for my dumb code, so he ought to
be aware. :)

dont-specialize-optional-args.diff
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