I just marked a whisper as the “solution” of the Playground topic in our forum.
Now people who can’t usually see whispers see it quoted in the first post.
Maybe that is intentional as theoretically the solution to a certain topic could be found in a whispered post, but in that case I think it would be better to have the option to un-hide that post and only then be able to mark it as a solution.
As I understand it - a whisper is only viewable by Staff - thus it would seem to be a bug that Whispers can be marked as solutions as in practice posts being marked as solutions are “support” questions and Staff whispers wouldn’t be seen…
I might have just confused the issue as well
I would say this is a bug, a minor bug that only staff can trigger, but yeah a bug
I just came across this myself - there seem to be a handful of things to keep in mind with whispers, and this is one of them. I was hoping to be able to use solved whispers as a part of my use of discourse as a ticket system. Often I want to mark tickets (discourse messages or topics with ticket tags) complete without informing the person the ticket is about…
[Accepted answer plugin] Add an ability to accept the initial (topmost) post as a solution could work for this but does not now.
An alternative to this seems to be making it possible to indicate whispers as the solution without exposing the whisper to non-staff. Maybe this is even a better solution in any case because then it forces us to actually write a note to explain the outcome. 
I did the same thing @tobiaseigen, and had marked several dozen that way before I realized the exposure! 