The only purpose of a code challenge or whiteboard session is to get the candidate and the interviewer talking about code. This is the only signal that you really want out of these exercises.
To do both is wasteful, but if I did either and that _didn't result in a conversation about code_, I wouldn't continue with the interview process. Their processes are broken.
That depends heavily on what they're actually screening for. I've received an embarrassing number of utterly atrocious code challenges from candidates in the past. If you can't code something like a roman numeral generator/reader (when allowed to do so at home, in whatever language you want, spending as long as you want on it) then continuing the interview process at this point is a huge waste of time.
In those processes, the purpose was to weed out people that simply could not code at all. Or even cheat, since there are solutions all over the internet for any language you want.
A later stage was to do some programming with us, and we'd talk through issues and see how they dealt with problems/etc.
Call it what it is then. It's not a challenge but a weed-out. I know full well that that's what your company is screening for, but that also means that I know that I don't want to work for you. My time and contributions will not be appropriately valued by such a company and I won't like working there. My skills/value are not commodity. I don't want to start out a hopefully multi-year relationship on dishonesty.
I imagine though that if you explicitly call it a weed out that a sizable percentage of the quality talent pool won't apply...unless they're a referral, and then why are they doing a code challenge anyway?
On that note: giving code challenges to employee referrals, especially hiring to their own teams, is a clear indicator that you don't trust your employees.
I guess I'm not really sure what the difference is, the entire point of all hiring processes is to "weed out" those that you don't want to hire. Otherwise what's the point?
> My time and contributions will not be appropriately valued by such a company and I won't like working there
I don't get how that's related.
> I don't want to start out a hopefully multi-year relationship on dishonesty.
There's no dishonesty there, the applicants were given a simple task to do in order to progress to the next step. If you're applying for a software engineer position and cannot even make a decent attempt at an extremely simple task when given complete flexibility then it's a waste of both of our time to continue.
(edit - I should point out that this was not at my current employer)
To do both is wasteful, but if I did either and that _didn't result in a conversation about code_, I wouldn't continue with the interview process. Their processes are broken.