There are rules which is good. They are complex which is not so good.
Personally I prefer a simple rule: If I define any constructors/assignment/move operators then I'll explicitly list the rest as either "= default" or "= delete" (as private, protected or public as apropriate) depending on what I want.
That way I won't have to worry about memorizing the complex rules (and getting them wrong in some subtle situation with hard to debug problems to follow) I'll just be explicit and tell the compiler "I don't care what you would have done, this is what I want".
Then it would not be possible to compile pre-C++11 code with the flag and get the benefits for free (as you suggested yourself!).
Similar rules already applied with the copy constructor and assignment and the rule of three was the guide to follow, now become rule of five/zero (
http://en.cppreference.com/w/cpp/language/rule_of_three) . The crux of it is do you need a hand in the class' resource management? If so, write copy/move/dtor yourself, else do not. The constructor(s) of the class should not be lumped in with the rest.
Yes, there are tricky scenarios where what is being generated makes for interesting puzzles. But i) avoid them ii) it's unlikely a mistake will result in a hard-to-debug problem; ie if you thought a move ctor was generated but is not, worst case is that the objects are getting copied instead, which should be noticeable in performance. Or if a copy ctor wasn't generated either, you 'll get a compile error.
