Why is this program erroneously rejected by three C++ compilers?

But it adds another question, should the specification lay out that much detail? Should we assume later that such interfaces might be used?

I find the "locked" and "closed" messages very interesting. It is a good indication of the type of culture the stack overflow community has. While I find this very funny, many people that run the community clearly want people to ask seriously questions and for people to give thoughtful and on topic answers (not to suggest that it is a bad thing).