Whyever **not** declare a function to be `constexpr`?

Question

Any function that consists of a return statement only could be declared constexpr and thus will allow to be evaluated at compile time if all arguments are

Answer 1

If the function has side effects, you would not want to mark it constexpr. Example

I can't get any unexpected results from that, actually it looks like gcc 4.5.1 just ignores constexpr