https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63645
--- Comment #12 from Markus Trippelsdorf <trippels at gcc dot gnu.org> --- 1) Your malloc is too small. It has to be sizeof (biggest member). So you're invoking undefined behavior. 2) In the if statement, where you probe the different members, you also invoke undefined behavior. So two bugs in one testcase.
