bug#63410: 30.0.50; [FR] Optionally allow defcustom to check type for standard value

[Ruijie Yu]

Similar to `setopt' introduced in 29, which warns when a variable gets
assigned a value with an incorrect type, I hope `defcustom' can
(optionally) check that the standard value of a customizable variable is
correct.  This would help external as well as internal package authors
to catch errors on the types.

I don't know how to define such optionality though -- can/should it also
be a customizable variable?  Or maybe it should just be a
function/command which checks customizable variables (e.g., a single
variable; a group; all of them; etc.), and warns about all the type
mismatches along the way?

In addition, a "check all variables within this group (and
subgroups)" might be helpful as well.

At the moment, the only via workaround I know of to check existing value
type for any given variable is this:

    (setopt variable variable)

But this introduces the runtime cost of the :set function, which
sometimes do pretty expensive things like (re-) loading a package.

Another workaround I know of, of course, is to run

    M-x customize-variable

on the variable, but that is painfully slow compared to the first
workaround.

If no-one gets to it first, I may start looking into how to implement it
within a week or two.

-- 
Best,


RY