https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892
--- Comment #45 from Andrew Haley <aph at gcc dot gnu.org> --- (In reply to Davin McCall from comment #44) > > Well, perhaps not, but this is the language specification. > > The "one special guarantee" clause appears in the section describing union > member access via the "." or "->" operators, implying that it only applies > to the access of union members via the union. I don't believe that's what is intended, or that you can make such a conclusion based on the section in which the rule appears. It applies to other accesses too, as is (somewhat) made clear by the rationale in http://www.open-std.org/jtc1/sc22/wg14/www/docs/n685.htm: The proposed solution is to require that a union declaration be visible if aliases through a common initial sequence (like the above) are possible. Therefore the following TU provides this kind of aliasing if desired: union utag { struct tag1 { int m1; double d2; } st1; struct tag2 { int m1; char c2; } st2; }; int similar_func(struct tag1 *pst2, struct tag2 *pst3) { pst2->m1 = 2; pst3->m1 = 0; /* might be an alias for pst2->m1 */ return pst2->m1; } I know this is non-normative and not even in the standard, but it does explain what was intended.
