https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892
--- Comment #44 from Davin McCall <davmac at davmac dot org> --- > 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. As has been pointed out by others, the guarantee is surely not meant to trump all other rules regarding access, so this is a reasonable interpretation (since otherwise, it is totally unclear when it does apply and what exactly "it is permitted" even means). Note that without that clause, type punning structs via a union would essentially be impossible (since layout is implementation defined or unspecified). The "common initial sequence" requirement is the only part of the standard which requires that structs with similar members have them layed out in the same order and alignment. Since this only matters for type punning, it again makes sense that this would be specified in the one section which actually allows for type punning (even if only in a non-normative footnote) - that is, union member access via a union. It's clear why it is needed for this, but to extend that to any access of union members (including not via the union) seems like a stretch. If that was intended, why isn't it specified in 6.5? The only thing that suggests an alternative interpretation to what I've described above is the requirement that the declaration of the completed type of the union be visible, which is redundant if the access must be via the union type. However, interpreting this to mean that the "special guarantee" applies globally is far more problematic than assuming that the requirement is just redundant.
