> Why do people react to C++ this way and not other languages?
Probably because stuff like boost::mpl (and so many awful half-implementations of it) exists.
Sadly, if there's a perceived use for a C++ feature, it will be used, no matter how convoluted it is. This is a specific case of a general method for adding state to constexpr, which might be considered useful... I wouldn't be shocked to see this used in the future.
If someone writes a "here is a cool thing" article for another language involving arbitrary obscure details they are praised with "cool hack". If the language is C++ then out comes all the rants about how C++ is inherently unmaintainable because of "this kind of thing".
Why do people react to C++ this way and not other languages?
The reason this happens is because the only thing all the other comments in this thread are really saying is "Preconceptions about C++ confirmed!". It's easy to reap the karma with such comments.