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

[Stefan Monnier]

> (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