If I understand the question correctly, applying enable_if to f itself like the following will solve the problem.
I can't answer your question ("why not?") but I can provide a work-around. But man, it's ugly. You end up defining a structure and a function for every function.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.