https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87363
--- Comment #2 from joseph at codesourcery dot com <joseph at codesourcery dot com> --- On Wed, 19 Sep 2018, msebor at gcc dot gnu.org wrote: > Other than that, since > > When a value is stored in a member of an object of union type, the bytes of > the object representation that do not correspond to that member but do > correspond to other members take unspecified values. > > the value of u.z is unspecified after the initialization of the union u in the > test case. No, that's only about those bytes that are outside the member that was stored (so if you have a union between int and double, and store in the int, the parts of the double after the initial sizeof (int) bytes become undefined, for example). Type-punning between union members (when you explicitly access using the union type), up to the lesser of the sizes of the two members in question, has been defined since C99 TC3 (in a footnote, so not normative, but the intent is clear).
