Calling non-static member function outside of object's lifetime in C++17

Question

Does the following program have undefined behavior in C++17 and later?

struct A {
    void f(int) { /* Assume there is no access to *this here */ }
};

int main(         


        

Answer 1

It’s true that trivial destructors do nothing at all, not even end the lifetime of the object, prior to (the plans for) C++20. So the question is, er, trivial unless we suppose a non-trivial destructor or something stronger like delete.

In that case, C++17’s ordering doesn’t help: the call (not the class member access) uses a pointer to the object (to initialize this), in violation of the rules for out-of-lifetime pointers.

Side note: if just one order were undefined, so would be the “unspecified order” prior to C++17: if any of the possibilities for unspecified behavior are undefined behavior, the behavior is undefined. (How would you tell the well-defined option was chosen? The undefined one could emulate it and then release the nasal demons.)