https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79167
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> --- N.B. https://gcc.gnu.org/projects/cxx-status.html shows that the "Refining Expression Evaluation Order for Idiomatic C++" feature (which changed these rules) isn't supported until GCC 7.
