[htilford]

The fact that it most people think it's a well-defined operation is what makes it a useful question. A total newbie who doesn't know how to code will fumble. A junior engineer will get to the simple solution. A senior engineer will ask you to define it better. Etc... It's an effective tool for determining level because there isn't a limit on how much depth you can go into about it.

[technion]

I'm jaded enough to imagine that if I ask someone to define it better and try to give a "senior" response that covered codepoints, an interviewer will write down "struggled with simple question".

Edit: That seems consistent with a statement made in another response.

[Doxin]

I feel like the best way to tackle questions like these is to first jot down a simple solution and then point out the various ways it doesn't work.