As of this writing (but perhaps not for very much longer!) the four mainstream compilers on Godbolt Compiler Explorer give four different answers for this simple C++ program:
G. Chari, D. Garbervetsky, and S. Marr. Proceedings of the 11th Workshop on Implementation, Compilation, Optimization of Object-Oriented Languages, Programs and Systems, page 5. (Jun 19, 2017)